Climate change skeptics used the recent slowdown in observed surface warming to claim that 20th century warming was temporary and that the Earth would return to lower average annual temperatures. They offered up many potential explanations for the slowdown, none of which make physical sense. The Sun’s 11-year cycle (often used to explain away warming), a primary argument brought forth, is not the reason: this cycle’s solar maximum is near at hand, yet warming has slowed down recently.
Recently accepted research points to a viable physical explanation. In addition to oceanic transport of heat to the deep ocean and recent La Nina events, sulfuric emissions from small and mid-sized volcanoes entered the lower stratosphere and reflected more incoming solar radiation than normal. This research separated the effect of natural sulfur emissions from anthropogenic emissions, using a model, to determine the former had a much larger influence than thought. Aerosol optical depth (AOD) is a calculated metric used to represent how opaque or transparent the atmosphere is to different radiation wavelengths. The layer between 20 and 30 km increased 4-10% per year since 2000, which is a significant change from normal conditions – significant enough to have effects on Earth’s climate.
Here is one of the paper’s graphical results:
As the caption says, satellite measurements are denoted by the thick black curve. Note the large increase in AOD (higher opacity) over the tropics in the mid-2000s (b) and the large AOD increase over the northern mid-latitudes in the late-2000s (a). While not a perfect fit to the observations, the model run with volcanic eruptions (red curve) does the best job of explaining the origin of the SO2. Individual eruptions are indicated by black diamonds on the bottom of each sub-plot. The effects of volcanic eruptions on climate are, in a general sense, well-known. Injections of SO2 into the stratosphere reflects sunlight, which reduces the amount of energy entering the Earth’s climate system. The difference between one large-scale eruption (e.g. Pinatubo in 1991) or many mid-sized eruptions in a short time-period (see above) is not large as far as the climate is concerned.
This could be good news as far as the climate is concerned, at least in the shorth-term. If incoming energy were reflected back into space instead of being stored in the system, we can physically explain the observed temperature trend slowdown (see Figure 2) and treat the slowdown as real instead of waiting for that energy to transfer from the oceans to the atmosphere, for example.
There is also bad news however. From the study (emphasis mine):