Effects of Global Warming on the Environment: Trends in Hurricane Frequency and Intensity

In all low latitude ocean basins except the South Atlantic and Southeastern Pacific, tropical cyclones develop during the warm seasons and pose hazards to shipping and coastal areas as they move slowly westward and poleward around the massive subtropical high pressure systems. The strongest of these storms are called hurricanes in the North Atlantic and East Pacific, typhoons in the western North Pacific, and cyclones in the South Pacific and Indian Oceans.  For tropical storms to form, they require (1) warm SSTs (26˚C (79˚F) or higher), (2) weak upper level winds and wind shears, preferably with diverging flow aloft, (3) a low level cyclonic (counterclockwise) circulation, and (4) atmospheric instability that leads to organized convective clouds (thunderstorms).  We will look at evidence regarding any recent change in frequency or intensity for these storms, and then discuss model projections for the latter part of the 21st century.

Tropical Storm Frequency

First, let’s look at the frequency of storms in the North Atlantic Basin.  Figure 1 shows the trend of Atlantic tropical storms from 1878 to 2006. [1] Prior to the advent of meteorological satellites in 1965, the number of storms would have been significantly underestimated without some correction, since the only oceanic observations came from sparse ship reports. The trend line was adjusted to estimate the number of missing storms, shown by the blue curve at the bottom of the figure. The resulting trend shows a slight average increase during that period (about +1.6 storms per century), although there were two minima during the 129 year period. The number of hurricanes from 1878 to 2006 seems to correlate with the observed increase in both global mean temperature and sea surface temperature (SST) (Figure 2). [2] However, there is no statistical significance to the trends in the other three curves: adjusted hurricane counts, U. S. landfalling hurricanes, and Atlantic SST relative to Tropical SST. Much of any increase in storm frequency has been determined to be due to increases in short duration (<2 day) storms that would likely have gone undetected in the early period of record. [3] Thus, there is much uncertainty in the trends in the Atlantic Basin, due mainly to the relatively short historical period of quality observations. The correlation is even weaker when we consider landfalling U.S. Atlantic hurricanes. Globally, the results are similar. A WMO report indicated that there was uncertainty that past trends in tropical storm occurrence exceeded the natural variability. [4]

  

Figure 1.Number of Atlantic Ocean Tropical and Subtropical storms from 1878 to 2006 adjusted prior to 1964 to account for undetected storms in the pre-satellite era (blue curve at bottom). (Source: National Hurricane Center (NOAA) HURDAT best track dataset)

Figure 2. Normalized trends of Tropical Atlantic indices for the period 1878-2011. Dashed blue lines for the top three panels show statistically significant trends. (adapted from Vecchi and Knutson, 2011)

Projections of tropical storm frequency through the 21st century were obtained using a High Resolution (50 km grid) Atmospheric Model (HiRAM) that incorporates projected changes in SST [5]. The model was downscaled into the Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Model following storm genesis to determine tracks and intensities. The results show a decrease in frequency of tropical storms globally by 6-34%. The only region showing an increase is the Eastern North Pacific (from the west coast of Mexico to Hawaii) (Figure 3). This seems counter-intuitive since SSTs are forecast to rise significantly, so we would expect tropical storm frequency to increase also. Possible reasons for the decrease are: (1) a weakening of tropical circulations that lead to the required thunderstorm activity, or (2) drier mid-tropospheric conditions which would also suppress convection.

Figure 3. Frequency of tropical storms projected for the late 21st century based on anticipated global warming and resulting SST increases. The difference (late 21st century minus present-day) is shown in the bottom panel. Except for the Eastern North Pacific, tropical cyclone frequency is expected to decrease. (From Knutson et al., 2015)

 

  • Tropical Storm Intensity

The intensity of tropical storms and hurricanes is measured using the maximum sustained wind speed, minimum sea level pressure (SLP), and something called the Power Dissipation Index (PDI) which combines cyclone strength, duration, and frequency. A comparison of the PDI for North Atlantic cyclones with concurrent SST is shown in Figure 4. [6] The PDI decreased to a minimum in the period from 1970-1980. Since that time, an uptick has occurred, followed by another decrease after 2005. It correlates pretty well with the trend of SST, except for the period 2008-2013.

Figure 4. Observed trend of the five-year mean Power Dissipation Index (PDI), a measure of tropical storm intensity, versus SST from 1951-2013. (2016 Update to data from Emmanuel, 2007 [6])

Tropical storms and hurricanes intensify when they move into areas of favorable conditions such as: (1) very warm SSTs that extend through deep layers of the ocean, and/or (2) weak winds aloft with pronounced divergent outflow. Unfortunately, short range prediction of intensification is poor compared to prediction of storm tracks. Long range climatic predictions of storm intensities must be based on assumptions about the model-simulated life cycles of storms under future climatic conditions.

Projections of the future intensity of tropical storms assuming greenhouse gas warming show increases of 2-11% by 2100. Coupled with this is an expected increase in the precipitation rate of about 20% within 100 km of the storm centers. [5] Considerable variations were observed regionally from basin to basin. As an example, Figure 5 shows the expected distribution of minimum Sea Level Pressure (SLP) for a scenario in which CO2 levels increase 1% per year for 80 years (heavy line) versus the current mean SLP distribution (thin line with open circles). The mean minimum SLP is about 10 millibars lower for the high CO2 scenario. More disturbingly, recent research suggests that if global warming occurs as forecast, extremely rapid intensification (>60 knots in 24 hours) of landfalling hurricanes in the United States  could occur once every 5-10 years as opposed to once every 100 years in the past.[8]

Figure 5. Simulated distribution of hurricane minimum Sea Level Pressure (SLP) for a high CO2 scenario possible after 80 years (thick line) versus current distribution.

In summary, recent trends in the frequency and intensity of tropical cyclones have not been significant enough to be certain that they exceed the natural variability, since the historical record is so short. Projections of future storm activity through the end of the 21st century based on numerical models portend a significant reduction in storm frequency but a likely increase in intensity, assuming further increases in SST due to higher levels of greenhouse gases. There is also the possibility that there will be more frequent occurrences of very rapid intensification prior to landfall in the United States, mostly along the Gulf of Mexico coastline. All this of course, is predicated on the continued rise of global mean air and sea surface temperatures. Although this is likely, the magnitude of the increase is not a certainty by any means.

References:

  1. Vecchi, G. and T. Knutson, 2008: On Estimates of Historical North Atlantic Tropical Cylcone Activity. Journal of Climate, 21, pages 3580-3600.
  2. ___________________, 2011: Estimating Annual Numbers of Atlantic Hurricanes Missing from the HURDAT Database (1878-1965) Using Ship Track Density, Journal of Climate, 24, pages 1736-1746.
  3. Landea, C., G. Vecchi, L. Bengtsson, and T. Knutson, 2010: Impact of Duration Thresholds on Atlantic Tropical Cyclone Counts. Journal of Climate 23, pages 2508-2519.
  4. Knutson, T., J. McBride, J. Chan, K. Emmanuel, G. Holland, C. Lansea, I. Held, J. Kossin, A. Srivastava, and M. Sugi, 2010. Tropical cyclones and climate change. Nature Geoscience, 3, pages 157-173.
  5. _______, J. Sirutis, M. Zhao, R. Tuleya, M. Bender, G. Vecchi, G. Villarini, and D. Chavas, 2015. Global Projections of Intense Tropical Cyclone Activity for the Late Twenty-First Century from Dynamical Downscaling of CMIP5/RCP4.5 Scenarios. Journal of Climate, 28, pages 7203-
  6. Emanuel, K.A. 2016 update to data originally published in: Emanuel, K.A. 2007. Environmental factors affecting tropical cyclone power dissipation. of Climate Vol. 20(22), pages 5497–5509.
  7. Knutson, T., and R. Tuleya, 2004: Impact of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization. Journal of Climate, 17, pages 3477-3495.
  8. Emmanuel, K, 2017: Will Global Warming Make Hurricane Forecasting More Difficult? Bulletin of the American Meteorological Society, Vol. 98, March, pages 495-
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Effects of Global Warming on the Environment: Heavy Precipitation and Flooding

Over the past century (1910-2010) the United States has observed an increase in the occurrence of heavy precipitation events, based on rain gauge data collected by the National Oceanic and Atmospheric Administration (NOAA). One measure of this is the percentage of land area in which a greater than normal percentage of total annual precipitation has occurred from single day storm events. This percentage has risen from about 8-10% between 1910 to about 1990 to roughly 15% since 1990. (Figure 1) [1] More evidence of this is the steady increase since the 1950’s in the number of 2-day precipitation events that are exceeded only once in a five year period.  The largest increases in precipitation seem to be in the Midwest and Northeast, while many parts of the West and Southeast have had decreases (Figure 2). The causes for this observed increase are believed to be the greater amount of water vapor that can be contained in the air as average temperatures increase, and the occurrence of more intense weather systems.

Naturally, one would think that as a result of more extreme precipitation events in the U.S., flooding, especially flash-flooding, would also show an increase. Even though stream flows in the U. S. have increased since the 1940’s, peak stream flows that would be associated with flooding events have not had a similar increase. Further research is needed to reconcile this difference, but research is handicapped by the short period of record for stream flow gauges (for example, only 30% of stream flow gauges have been in existence beyond 25 years in the south and Middle Atlantic regions).

heavy-precip-extremes_2016_epa_noaa

Figure 1. Percent of land area in the 48 contiguous states affected by extreme one-day precipitation events from 1910-2015.

precipitation_change_map_2015

Figure 2. Percent change in precipitation in the United States from 1901-2015. (Source: NOAA 2016)

 

A rising trend in total precipitation has also been observed globally over land since 1901 using surface observations. (Figure 3) [2] A shorter period of observations derived from both microwave and infrared satellite data used for the Global Precipitation Climatology Project (GPCP) has shown a similar increasing trend in precipitation over the wettest regions of tropical oceans (30N to 30S) since 1979, while drier Subtropical oceanic regions have seen the opposite trend. [3] Over tropical land areas however, there was no significant tendency. A more recent study that used precipitation reports from a long period (1930-2005) of island surface observations showed good agreement with the satellite (GPCP) studies over tropical oceans. [4]

global-precipitation_trends_2015_epa

Figure 3. Precipitation anomalies (inches) over land worldwide (1901-2015).

 

Projections of precipitation extremes through the year 2100 have been obtained from a global 20 km general circulation forecast model, assuming an approximate doubling of the CO2 concentrations during the period. The results show that many regions that are currently wet (i.e., South Asia, the Amazon, West Africa, and the northeast U.S.) will experience a significant increase in heavy precipitation, while regions that are currently dry (South Africa, south Australia, and the southern Amazon) will suffer even further decreases in rainfall. [5] (Exceptions are extremely dry areas such as Antarctica, Sahara, and Tibet where the length of dry spells will decrease). These results are consistent with the trends seen over the last century in surface observations and more recent (since 1979) satellite data described above.

Recent analysis of trends in inundation frequency for four National Weather Service flooding categories using twenty years of data from more than 2,000 river gauges suggests that flooding will increase in the northern U. S., while decreasing in the southern U. S. [6]. Another source of data used in the study is from a NASA satellite that measures ground moisture or “basin wetness.”

In summary, while there will be significant regional differences in the observed response of precipitation to an increase in global temperatures (and thus, SST), it is expected that there will be an increase in rainfall extremes through the end of this century. These tendencies have already been observed in surface and satellite observations within the past 100 years.

References:

  1. U. S. Environmental Protection Agency (EPA), 2016: Climate change indicators in the United States, 2016. Fourth edition. EPA 430-R-16-004. http://www.epa.gov/climate-indicators
  2. Takahashi, K. and co-authors, 2006: Trends of Heavy Precipitation Events in Global Observations and Reanalysis Datasets. Scientific Online Letters on the Atmosphere (SOLA), Vol. 2, pages 96-99.
  3. Allan, R. P., B. Soden, V. John, W. Ingram, and P. Good, 2010: Current changes in tropical precipitation. Envoronmental Research Letters, 5 (2010), 7 pages.
  4. Polson, D., G. Hegerl, and S. Solomon, 2016: Precipitation sensitivity to warming estimated from long island records. Environmental Research Letters, 11 (2016), 8 pages.
  5. Kamiguchi, K., and co-authors, 2006: Changes in Precipitation-based Extreme Indices Due to Global Warming Projected by a Global 20-km-mesh Atmospheric Model. Scientific Online Letters on the Atmosphere (SOLA), Vol. 2, pages 64-67.
  6. Slater, L. and G. Vallarini, 2016: Recent trends in U. S. flood risk. Geophysical Research Letters. 43. doi:10.1002/2016GL071199

Next: Trends in Tropical Storm Frequency and Intensity

Drought Cycles and Crop Yields

There is evidence that suggests that certain areas of the globe have had more pronounced and longer lasting droughts over the past century. [1] The effects of increases in atmospheric CO2 have been shown to have a greater effect on temperature in arid regions such as the southwestern United States, the African Sahel, Australia, South Africa, and the Arctic. Over the past several decades, these regions have warmed more significantly that the rest of our planet, as shown by the table below from the 2001 IPCC report. From 1976 to 2000, warming in desert areas ranged from 0.5-2°C, which is much larger than the average increase of 0.45°C for the entire planet (as of 2000). The amount of warming predicted under two different CO2 increase scenarios (see table) for the latter part of the 21st century (2071-2100) is significant for all deserts.

Precipitation changes are more complex, however. Rainfall has decreased since 1976 in many deserts. The predicted rainfall trends for two scenarios are shown in the table to either increase or decrease, depending on locations. An important factor seems to be the season in which the desert normally receives rainfall.  Desert areas where summer rainfall is prevalent have dramatically less precipitation, while the opposite is true for deserts with winter rainfall. The reason for this is believed to be the expected increase of El Nino events (described earlier) which enhance cool season rainfall.

Desert_warming_IPCC_2001_table2.1

Recent findings from at least two studies indicate that there has been an increase in desert vegetation due to (1) an effect known as “CO2 fertilization,” and (2) an increase in moisture and thus, precipitation due to the warmer temperatures. [2] The “greening” has been observed by environmental satellites with sensors that detect chlorophyll associated with plant life, and was supported by computer model calculations. This represents an important feedback mechanism not previously accounted for. It is estimated that 16 out of the 20 most important crops will benefit from increased CO2.

A more recent study that also used satellite data from 1982-2009 (Figure 1) found that while a small area of the Earth (<4%) experienced leaf loss (browning), 25-50% of the global surface showed a greening during growing seasons. [3] By comparing the results with numerical ecosystem model simulations, it was estimated that 70% of the greening was likely due to “CO2 fertilization,” while 9% was due to increased nitrogen.

Zhu_etal_2016_LAI_greening

Figure 1. Change in percentage of vegetation cover (expressed as Leaf Area Index (LAI)) from 1982-2009 derived from NASA’s MODIS instrument on polar-orbiting satellites. Observed LAI was compared with a 28-year average LAI assuming IPCC scenario S1. (From Zhu et al. 2016)

In the crop-growing regions of the world, droughts and resulting crop failures are strongly related to the short range El Niño and La Niña cycles. But additional effects due to global warming could produce added stress, according to computer models that assume a rise of about 1.8°C by 2030.[4]  The most hard-hit areas are expected to be southern Asia and southern Africa where principal crops (wheat and corn) may be reduced by 10 to 15%.  Unfortunately, these are among the most poverty-stricken regions of the world.  One unknown variable in this projection is rainfall. The computer models don’t agree on how rainfall will change in the coming decades, nor do they consider extremes in temperature and rainfall, which create major stress on plants. Developing drought-resistant varieties of crops or growing different crops entirely would help alleviate this concern.

Warming temperatures, if they occur as predicted, will also likely lead to an increase in insects and plant diseases, which are certain to reduce crop yields in many areas. Increasing CO2 will have some positive effects on crop yields however, helping to compensate for effects of rising temperature. In addition to being an essential compound in photosynthesis, more CO2 will help plants by increasing their efficiency in the use of water.

In summary, the effects of predicted global warming on drought cycles and crops will be mixed, resulting in more pronounced drought and crop failures in some areas, and increased rainfall in others, including some desert areas where winter rains are relied upon. However, predictions of cataclysmic drought and crop failures seem to be contradicted by recent satellite observations of increased plant area coverage globally over the past few decades due to the positive effects of CO2.

References:

  1. Intergovernmental Panel on Climate Change (IPCC) Report, 2001:
  2. Donohue, R., M. Roderick, T. McVicar and G. Farquhar, 2013: Impact of CO2 fertilization on maximum foliage across the globe’s warm, arid environments. Geophysical Research Letters, Vol. 40, pages 3031-3035.
  3. Zhu and co-authors, 2016: Greening of the Earth and its drivers. Nature Climate Change, doi:10.1038/nclimate3004.
  4. Lobell, D. B., Burke, M. B., Tebaldi, C., Mastrandrea, M. D., Falcon, W. P., Naylor, R. L., 2008: Prioritizing climate change adaptation needs for food security in 2030. Science, Vol.319 (5863): 607-610

Next: Heavy Precipitation and Flooding

Variations in Solar Output and their Effects on Climate

Our sun is a small star with remarkably stable energy output.  While many stars pulse dramatically in size and brightness, our sun’s luminosity varies only about 0.1% over the course of its 11-year solar cycle (Figure 1). However, even this small change in incoming radiation exceeds all the other energy sources (such as radioactivity from the earth’s core) so it is important to consider.  [1]

tsi_composite_strip

Figure 1. Strip chart showing Total Solar Irradiance (TSI) (top) from several sources, and the number of sunspots (lower) since 1975. (Source: University of Colorado)

The changes in extreme ultraviolet (EUV) radiation can be particularly important, varying by a factor of 10 or more. [1] EUV can affect the Earth’s upper atmosphere during solar maxima by creating more nitrogen oxides (NOx) which in turn can reduce the ozone levels by a few percent, resulting in more UV radiation reaching the surface. The effect of the UV increase on our weather and climate is complicated, but appears to have an effect on certain regions, such as the Pacific Ocean Basin, more than others. In the Pacific, a La Nina-like pattern seems to occur at the solar maxima, with cooler SSTs in the East Pacific. Solar cycle forcing seems to affect the general circulation more than having a direct temperature signal, with an increase in precipitation the most likely outcome.

One major connection however, was the so-called “Maunder Minimum” of the late 1600’s and early 1700’s (Figure 2), which was the coldest part of the “Little Ice Age.” [2] This period was characterized by bitterly cold winters in Europe and North America, although there wasn’t actually any glaciation. The solar cycles during this period were extremely weak based on the numbers of sunspots observed (Figure 2). The number of sunspots has risen steadily since the late 1600’s which many scientists believe has contributed to as much as half of the 0.6°C  temperature increase since 1900.  [3] However, there is still much uncertainty in these estimates.

Maunder_minimum

Figure 2. Variation in sunspot frequency since the year 1600.  The “Maunder Minimum” was accompanied by very cold winters in the Northern Hemisphere. (Source: NASA Marshall Space Flight Center)

It is possible that we are now entering a mini-version of the Maunder Minimum, as solar activity is the weakest it has been in more than 50 years, and is predicted to reach a pronounced minimum by 2020 (Figure 3). Other models suggest a minimum somewhat later, in the 2030’s. Research also seems to suggest that there is an inverse correlation between the length of sunspot cycles and the magnitude of the Earth‘s warming or cooling during the next sunspot cycle (i.e., longer cycles = cooler average temperatures) (Figure 4). [4] Based on this research, a drop in average temperature during current solar cycle number 24 (ending 2020 or later) of about 1˚ C is possible for parts of the globe. Obviously, this has not happened yet, so time will tell if this prediction will come true.

Solheim_etal_sunspot forecast

Figure 3. Recent frequency of sunspots since 1995, along with NASA prediction out to 2020. (NASA MSFC)

Solar cycle length vs warming

Figure 4. Length of sunspot cycle (years) versus global temperature change (C) since 1860. The length axis at right is inverted.

In summary, although the Sun’s energy output has been reliably steady, there is historical evidence for relatively cold periods caused by reduced solar luminosity. The frequency of sunspots and the lengths of sunspot cycles seem to have some relationship to global temperatures. An increase in the number of sunspots may have contributed to a portion of the observed global temperature increase over the past 150 years. Finally, forecasts of solar activity suggest that some cooling may occur in the next two decades as a result.

 

References:

  1. Philips, T. 2013: Solar Variability and Terrestrial Climate. NASA Science News, Jan. 8, 2013, available at: http://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/
  2. Hathaway, D. 2016: The Sunspot Cycle, available at: http://solarscience.msfc.nasa.gov/SunspotCycle.shtml
  3. Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report Climate Change, 2007: Chapter 6. Radiative Forcing of Climate Change
  4. Solheim, J-E, K. Stordahl, and O. Humlum, 2012: The long sunspot cycle 23 predicts a significant temperature decrease in cycle 24. Journal of Atmospheric and Solar-Terrestrial Physics. Vol. 80, pages 267-284.

Next: Effects of Global Warming on the Environment: Drought Cycles and Crop Yields

Short Period Natural Climate Variations

Over shorter time periods, there are important natural cycles in the sea surface temperatures (SSTs) that result in major atmospheric changes over multi-year and multi-decadal time scales. Examples of this are the well-known El Niño (the warm phase of the El Niño-Southern Oscillation (ENSO)) and its alter ego La Niña (ENSO cool phase), that occur every 3-5 years on average. The Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO) occur in 10-50 year cycles. Most of these phenomena have only been discovered within the last 30 years, so there is much we have to learn about them.  Their effects are complex, and can affect many areas of the globe. Although the effects on weather and temperatures are mostly found in certain preferred regions, they may also create an imbalance in the global climate system. Thus, a strong, long-lasting El Niño or La Niña for example, can affect the mean global temperatures.

  1. El Niño and La Niña

El Niño occurs when there is a reversal in the low level winds in the Tropical Pacific Ocean, resulting in westerly (west to east) flow instead of the normal easterly trade winds.  The root cause for this phenomenon is not well understood. The abnormal westerlies cause the warmer waters of the central and western Pacific to gradually move toward South America, leading to higher than normal SST (Figure 1). This results in more moist, unstable conditions along the west coast of South America (Peru, Ecuador, and Chile), leading to greater precipitation than normal and even severe flooding. It affects the local economy also because the fishing industry suffers due to a lack of nutrients in the ocean, causing the fish to go elsewhere. Due to feedback from the lower levels into the upper atmosphere caused by deep convective clouds (thunderstorms), El Niño produces other effects on a global scale that include: (1) a higher frequency of tropical storms and hurricanes in the Eastern North Pacific between Mexico and Hawaii, (2) lower hurricane frequency in the Atlantic, (3) wet winters in the southern United States, and (4) increased drought in Australia.  El Niño usually results in warmer than normal average global temperatures, which has certainly been the case in 1998-99 and 2015-16.

ei_nino

Figure 1. Surface wind differences from normal (arrows) and sea surface temperatures (SST) associated with an El Nino (left) and La Nina (right) pattern. Arrow lengths are proportional to wind speed. (Source: NASA Jet Propulsion Laboratory)

La Niña is just the opposite situation. Stronger than normal easterly trade winds (right panel of Figure 1) during a La Niña result in observations of much cooler than normal (3-5°C) SSTs in the tropical eastern and central Pacific Ocean. This leads to: (1) drought along the coasts of Peru and Chile, (2) more precipitation in the northern U. S. and Canada, (3) drier in the southern U.S., and (4) more frequent hurricanes in the North Atlantic Ocean basin. La Nina usually results in cooler than normal average temperatures for the U.S. See Figure 2 for a summary of the effects of both El Nino and La Nina.

El Nino La Nina conditions

Figure 2. A summary of conditions that accompany El Nino and La Nina in North America and the North Pacific Ocean.  (NOAA/NCEP/NWS)

The frequency of both El Niño and La Niña is irregular, but there are usually two or three of each within the course of a decade. Duration is short, lasting no more than 18 months. There is some evidence that El Niños have become more frequent within the last 50 years,[1] while La Niñas have become less frequent. It is not known if this is a random occurrence or is somehow related to climate change. Climate models predict an increase in the frequency of extreme El Niños by a factor of two.

2. Pacific Decadal Oscillation (PDO)

A longer period but equally important phenomenon discovered in 1997 is the Pacific Decadal Oscillation or PDO, marked by alternating colder and warmer than normal SSTs in the Northeast Pacific north of 20°N.  During a warm (positive) phase, the northeast Pacific warms while the western Pacific cools (left panel of Figure 2).  The opposite occurs during a cool (negative) phase (right panel of Figure 2). As with the ENSO, the cause of PDO is not known. The effects of the PDO on temperature and precipitation are pronounced in the northeast Pacific, northwest U. S., Canada, and Alaska. Analysis of tree ring data back to the year 993 shows that the PDO has significant year-to-year variability but a major period interval of 50-70 years.[2]  Since we have been in a cool phase since the mid-1990’s, there is evidence that this has contributed to the “pause” in global warming observed since that time.[3]  The theory is that if we return to a warm phase, the warming trend in global temperature may resume.

pdo_warm_cool

Figure 2. Sea surface temperatures (SST) and surface winds associated with the warm phase (left) and cool phase (right) of the Pacific Decadal Oscillation (PDO). (Source: NASA JPL)

References:

1. Cai, W., et al., 2014: Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change, Vol. 4, 111-116.

2. MacDonald G. M. and R.Case, 2005: Variations in the Pacific Decadal Oscillation over the past millennium. Geophys. Research Letters, Vol. 32, L08703, 4 pages.

3. Kosaka, Y. and S-P. Xie, 2013: recent global-warming hiatus ties to equatorial Pacific surface cooling. Nature, vol. 501, pages 403-407.

Next: Variations in Solar Output and their Effects on Climate

Looking at Climate in the Distant Past

Conclusions about the current state of the climate are based on a relatively short time span of 100 to 150 years of data from instrumented global observing sites. Some nations in the Northern Hemisphere (especially in Europe and North America for example) have local records that span a longer period from which we can infer important global climate events. The year 1816 for example, was known as the “year without a summer,” resulting in vast food shortages. This cold period was believed to be caused by the eruption of Mt. Tambora in Indonesia and other volcanoes in that region, leading to volcanic ash layers that reflected much of the sun’s incoming rays.[1] There is also evidence for a “Little Ice Age” between 1500 and the 1700’s marked by much cooler than normal temperatures (although there was never really any glaciation during that time).[2]

Climate information for a much longer period (hundreds of thousands of years) can be determined by analysis of “proxies,” or data sets from which temperature and other variables can be inferred using isotope analysis or other techniques.  Examples of proxy data include: tree rings, ice cores from glaciers, lake or ocean bed soil cores, and even stalagmites from caves.

Analysis of ice cores from the deep Antarctic ice sheets at the Russian Vostok research station has yielded climate information back 400,000 years or more. [3] The ice cores at this location were around 3300 meters (10,000 feet) deep.  Each layer of ice can show the amount of snowfall accumulation (and melting) during a year’s time, but more than that, it can tell us something about the CO2 and CH4 concentration of the atmosphere (parts per million – ppm) by analyzing air bubbles trapped in the ice. The accuracy is believed to be within a few ppm. Also, the average atmospheric temperature around the time each layer of ice was created can be estimated from the concentration of deuterium, (also known as heavy water) an isotope of hydrogen. Since the air bubbles are several thousand years younger than the ice surrounding them, a correction must be made to compare the timelines accurately. [4]

The results of their analysis reveal an astonishing cycle of four distinct cold, glacial episodes, separated by shorter interglacial periods about 100,000 years apart (Figure 1). The swings in global mean temperature during these episodes have been as much as 10 degrees C. During each inter-glacial period, the earth’s atmospheric temperature warms, and CO2 and CH4 concentrations rise. And of course, the glaciers recede in area over a period of 20,000 years or more. You can see from the graph that we are now in an inter-glacial period that began about 12,000 year ago, known as the Holocene epoch. This accounts for a natural occurrence of many of the conditions we observe today. Similar results were obtained for even deeper ice cores in another location in Antarctica (Dome C) that extends back to 800,000 years, and from dozens of deep sea bed cores [5].

Vostok_420ky_graphs

Figure 1. Change in CO2 concentration (blue), surface temperature (red), methane concentration (green), and solar insolation with deuterium concentration (orange) over the past 450,000 years. (From Barnola, et al. 1987)

Another important finding from the ice core studies was that the increases in global temperatures seem to precede slightly the rises in atmospheric CO2 concentrations, not vice versa. This suggests that increases in CO2 prior to each interglacial period did not cause the warming, but occurred as a result of the warming. How can this happen? When the ice and permafrost melted and the oceans warmed through natural causes, CO2 in vast quantities was released into the atmosphere.

Recent research has studied the ratio of oxygen isotopes from rainwater that deposits as minerals in stalagmites in a Chinese cave and found a relationship with precipitation amounts that extends back for a period of 640,000 years. [6] The intensity of the Asian monsoon can thus be estimated for a much longer period (nearly 300,000 years) than previously found in earlier studies. They found that pronounced dry, weak monsoons were observed with the termination of the last seven glacial periods, coincident with a rise in sea level as glaciers began to melt rapidly.

What caused these pronounced, periodic, natural changes in the Earth’s temperature, CO2 and CH4 compositions, ice coverage and even precipitation?  They are believed to be due to what are known as Milankovitch cycles, named after a Serbian astronomer who developed his theory in the 1920’s. [6] These are slow changes in the Earth’s orbit that bring our planet slightly closer to the Sun (thus bringing more insolation to the atmosphere) during interglacial periods and farther away during glacial episodes (Figure 2). The earth’s orbit is not circular but slightly elliptical, more like the shape of a fat egg. Changes in the orbital shape are due to the slight gravitational pull by the largest planets, Saturn and Jupiter that cause the orbit to rotate.  The tilt of the Earth’s axis relative to the orbital plane (which gives us our seasons) also varies with time and rotates, somewhat like a spinning top (see Figure 2).  The interaction of all of these effects is complicated, leading to major observed peaks every 100,000 years, and weaker ones every 20,000 and 40,000 years.

milankovitch_cycles

Figure 2. Depiction of the Milankovitch cycles in the Earth’s orbit and tilt. The eccentricity of the Earth’s orbit is exaggerated for illustration. (Source: University Corporation for Atmospheric Research)

What effect does the current man-made increase in greenhouse gases such as CO2 have on the natural glacial and interglacial cycles? Recent research suggests that anthropogenic effects could lengthen the current inter-glacial period to as much as 50,000 years. Certainly, this is a good thing for our future descendants!

In summary, looking at the Earth’s climate in the distant past helps to put our current situation in better perspective.  The melting of the glaciers and sea ice, warming temperatures, and CO2 increases are to be expected from natural processes during this current inter-glacial epoch.  Human activity adds to these conditions, resulting in record levels of CO2 in recent years, but it does not initiate them.

(Updated Oct. 2, 2016)

References:

  1. The Year without a Summer. Wikipedia, source: https://en.wikipedia.org/wiki/Year_Without_a_Summer
  2. Little Ice Age. Wikipedia, source: https://en.wikipedia.org/wiki/Little_Ice_Age
  3. Barnola, J-M., Raynaud, D., Korotkovich, Y. and Lorius, C., 1987: Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329, pages 408-414.
  4. Barnola, J-M., P. Pimienta, D. Raynaud, and Y. Korotkevich, 1991: CO2-climate relationship as deduced from the Vostok ice core: a re-examination based on new measurements and on a re-evaluation of the air dating. Tellus, 43B, pages 83-90.
  5. Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, et al. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present.Nature453: 379-382. 
  6. Lee, J., 2012: Milankovitch cycles. Encyclopedia of Earth. Source: http://www.eoearth.org/view/article/154612/

Next: Short Period Natural Climate Variations

Accuracy of Long Range Global Temperature Forecasts

 

Long range predictions of the expected increase in mean global temperature due to increases in greenhouse gas emissions (referred to as Equilibrium Climate Sensitivity (ECS)) during the 21st century vary from one or two degrees Celsius (C) to 4°C or more, [1] depending on the Global Climate Model (GCM) used and the assumptions regarding future greenhouse gas concentrations (Figure 1). 

GCM_forecasts_IPCC_2013

Figure 1. Future scenarios of global average surface temperature (°C) out to 2100 based on Global Climate Models (GCM) from the 2013 IPCC report. Solid lines show mean temperatures while shaded areas show uncertainties (5% to 95%). Intermediate scenarios are shown by the vertical shaded bars at right. The numerical values are the number of GCMs used to obtain the results.

The confidence in the larger temperature forecasts (>4°C) is low due to the fact that the performance of GCMs since the 1990’s has been poor.  A recent Canadian study shows that the projected warming trend from the early 1990’s to 2012 was twice as high as actually observed for 37 models in 117 separate forecasts (Figure 1).[2] When projected over even longer time periods, such as through the end of the 21st century, this could result in significant errors.

 

GCM_forecasts_1993-2012_Fyfe_etal_2013

Figure 1. Forecasts of temperature change (°C per decade) by GCMs (gray bars) versus observed change (red hatched area) from 1993 to 2012. (From Fyfe et al. 2013)

GCMs simulate the atmospheric motion and energy transfers over long periods of time. The overestimates of global mean temperature by GCMs are likely due to inaccurate assumptions about the: (1) reflection of incoming solar radiation, and (2) blocking of outgoing infrared radiation. [2,3] These two factors are mostly a result of cloud cover, which the models assume will shrink in area with rising temperatures, while the opposite is often the case. Increasing cloud cover can be caused by increased evaporation from ocean surfaces in a warmer climate, even in the Polar region as ice sheets melt.  

Another important mitigating factor is air pollution and volcanic aerosols, such as SO2, which help cool the planet by reflecting solar UV energy back into space.  Even lower atmospheric dust from the Sahara and other deserts is not simulated well in GCMs. [4] These dust clouds reduce heating of the sea surface, which affects the formation of tropical cyclones in the Atlantic and other ocean basins.

While improvements in the models are likely to occur, the result in the meantime is a wide range of projected temperature increases that will likely exceed the actual rate of global warming during this century as a result of the previous discussion. One researcher, the late Dr. William Gray, argued that the expected increase in CO2, even a doubling of CO2, will not bring the anticipated increase in global temperatures. Instead, he believed that warming of only half a degree C or less will occur, not the 2-4°C or more expected by the IPCC.[3]  This claim has historical basis. Around 1900, Swedish physicist Knut Angstrom determined that CO2 concentration beyond about 50 ppm has little effect on the Earth’s temperature, although the results of that experiment were disputed. Dr. Gray believed that changes in the strength of the inter-ocean circulation (such as the Atlantic Thermohaline Current) which controls the salinity of the oceans has led to the 0.8°C increase we have seen in the past century, not man-made greenhouse gases.

So based on the evidence at hand, a global warming scenario on the low end of the prediction scale, say in the range of an additional 1-2°C or so is what will most likely occur by the year 2100. Note that likely scenarios from the 2013 IPCC report have been reduced from those in earlier year reports, and one of them (RCP2.6) indicates less than 1°C of global warming. A recent study [5] affirms that the probability of the Equilibrium Climate Sensitivity (ECS) exceeding 4.5 C by the end of the 21st century is less than 1%. Likewise, the probability of an extremely low (1.5 C) value is less than 3%.  A central estimate of 2.8 C was given by Cox et al., with a confidence of 66%. Improvements in greenhouse gas emissions that are already underway could see a further reduction in this scenario.

References:

  1. IPCC Fifth Assessment of Climate Change 2013 – Synthesis Report
  2. Fyfe, J. C., N. Gillett, and F. Zwiers, 2013: Overestimated global warming over the past 20 years. Nature Climate Change, Vol. 3, pages 767-769
  3. Gray, W. M., 2012: Physical Flaws of Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver. Heartland Institute’s 7th International Conference on Climate Change (ICCC-7), Chicago, IL, 21 to 23 May 2012, available at: http://tropical.atmos.colostate.edu/Includes/Documents/Publications/gray2012.pdf
  4. Evan, A. T., C. Flamant, S. Fiedler, and O. Doherty, 2014: An analysis of Aeolian dust in climate models. Geophysical Research Letters, Vol. 41, pages 5996-6001.
  5. Cox, P., C. Huntingford, and P. Williamson, 2018: Emergent restraint on equilibrium climate sensitivity from global temperature variability. Nature, 553, 319-328.

Next: Looking at Climate in the Distant Past