A recent slow rise in the Earth’s mean temperatures observed by surface reporting stations since 1979 has been supported by microwave satellite data processed by NASA (Figure 1) that obtains an estimate of lower atmospheric temperature from the emission of thermal energy by oxygen molecules. Originally, it was thought that the satellite data did not show any warming, but after some recent corrections in the conversion of this data to temperature, the trend fell nearly in line with surface temperature readings for the same period. In Figure 1 it can be seen that since the late 1990’s the global mean atmospheric temperature was nearly steady, but has begun to increase again from 2015-2017, likely due to a strong El Niño event such as that which occurred in 1998. In fact, the years 2014 and 2015 were considered global records for the modern instrumented period (1850 to the present). The total warming during the period of satellite measurements has been about 0.8°C.
Figure 1. Mean temperature difference of the global lower atmosphere (as of March 2018) from the 1981-2010 average (degrees C) as measured by polar-orbiting environmental satellites. The red line is a smooth version of the data obtained by calculating a running 13-month average. (Source: University of Alabama-Huntsville)
There has been considerable debate about this recent “pause in global warming” from about 1998 to 2014. Some contend that it doesn’t really exist at all, or that the global temperatures really increased during the period, but at the rate of only 0.01°C per year, which is not at all significant. A recent paper went so far as to re-analyze historical temperature data from ship reports to remove perceived biases, resulting in a steady temperature rise during that period.  Ships measure the temperature of intake water used for ballast and engine coolant. However, since the depth at which this water originates varies from ship to ship depending on size and weight, it must be adjusted but wasn’t in the study.
Finally, sound judgement prevailed in a recent consensus article stating that the slowdown in global warming was real, and was believed to be caused mainly by a cool phase in the slow moving North Pacific ocean currents, referred to as the Pacific Decadal Oscillation (PDO) (see a later blog on this topic). This has resulted in excess atmospheric heat being absorbed into the oceans, leading to a rise in sea levels. They insist that “human-caused warming is continuing, but it is masked by natural factors.” It should be noted however, that absorption of heat and CO2 into the oceans is a normal process that goes on continually.
A recent, plausible theory for this pause in global warming was the discovery that aerosols in the lower stratosphere (above 8 km) have increased during the period from 2005 to 2012 due to volcanic eruptions.  The measurements were made on board instrumented commercial jets and by the CALIPSO satellite which is equipped with a earth-pointing lidar system. The volcanic aerosol which effectively scatters solar radiation into space is sulfur dioxide (SO2) gas. A similar, more dramatic event occurred 200 years ago when the eruption of Tambora volcano in Indonesia resulted in a “year without a summer” in 1816.
Was this pause in global warming predictable? A recent statistical study finds that it was not, due to the inability to accurately predict changes in the North Pacific sea surface temperatures (SST), a primary cause, and North Atlantic SST, a secondary driver. 
In short, satellite measurements support a gradual rise in global mean surface temperature (0.8 °C) since the late 1970’s, and little or no rise in global mean temperature from 1998 to 2014. A recent spike in temperatures may be partly due to a strong El Niño that has faded by mid-2016.
- Spencer, R. W., J. Christy, and W. Braswell, 2015: Version 6.0 of the UAH Temperature Dataset Released. (Available at: http://www.drroyspencer.com/2015/04/version-6-0-of-the-uah-temperature-dataset-released-new-lt-trend-0-11-cdecade/)
- Karl, T. R., A. Arguex, B. Huang, J. Lawrimore, J. McMahon, M. Menne, T. Peterson. R. Vose, and H-M Xhang, 2015. Possible Artifacts of Data Biases in the Recent Global Surface Warming Hiatus. Science, Vol. 348, issue 6242, pages 1469-1472.
- Fyfe, J. C., G. Meehl, M. England, M. Mann, B. Santer, G. Flato, E. Hawkins, N. Gillett, S-P Xie, Y. Kosaka, and N. Swart, 2016. Making Sense of the Early-2000s Warming Slowdown, Nature Climate Change, 6, pages 224-228.
- Andersson, S. M., Martinsson, B. G., Vernier, J-P, Friberg, Brenninkmeijer, C., Hermann, M., van Velthoven, P. F. J., and Zahn, A. Significant radiative impact of volcanic aerosol in the lowermost stratosphere. Nature Communications, 2015; 6: 7692 DOI: 10.1038/ncomms8692
- Mann, M. E.,B. A. Steinman,S. K. Miller, L. M. Frankcombe, M. H. England, and A. H. Cheung (2016), Predictability of the recent slowdown and subsequent recovery of large-scale surface warming using statistical methods, Geophys. Res. Lett., 43, 3459–3467, doi:10.1002/2016GL068159