Much has been made in the last year about the so-called global warming hiatus seen in sea surface temperatures:
You can see a consistent rise in land-ocean temperatures since the 60’s that mysteriously plateaus after around 2000. Although I have previously blogged about how this hiatus doesn’t mean that global warming isn’t happening, it’s still an oddity that climate scientists want to explain. Recent work has done much to explain this plateau, pointing to internal variability in the Earth’s climate system, lower solar radiance in the most recent solar cycle, changes in water vapor levels in the atmosphere, and increased anthropogenic sulfate aerosols released into the atmosphere that reflect sunlight. Now there’s more news…
Research recently published in Geophysical Review Letters points to myriad small volcanic eruptions and several larger ones releasing sulfur dioxide (SO2) into the air as a strong predictor of the warming hiatus, accounting for more of the temperature variability than previously believed. SO2 is a colorless gas, but you can imagine it billowing out with all the ash seen in the photo above! SO2 wanders into the atmosphere before resting in the stratosphere, where it reacts with water to form sulfuric acid (H2SO4), which happens to reflect incoming sunlight. This reduces the amount of radiation absorbed by the Earth, leading to less long wavelength emission from the Earth that can be absorbed by CO2 in the atmosphere to lead to global warming.
So how did the authors determine the relevance of volcanic eruptions? Previous research had hinted at the role of volcanic eruptions by connecting them to increased temperatures in the lower troposphere, the lowest part of the atmosphere about 10 km above the ground.
The current study went beyond this initial correlation to see if they could find a direction connection between eruptions and other climatic variables, including sea surface temperatures, mid- and upper-troposphere temperature, water vapor concentrations, and precipitation. Understanding all these relationships together provides a much more comprehensive picture of the volcanic effect. To do this, the researchers used an existing database taken from satellite measurements that give sulfate aerosol concentration levels in the stratosphere. The authors analyzed data taken from 1985-2013 using monthly means and zonal averages in 5 degree latitude increments.
So what’s the final word? Volcanic eruptions have definitely increased aerosol concentrations in the same years that the global warming hiatus began! The main result is shown in the figure below:The x-axis in both graphs is the year, increasing from left to right. The y-axis, the optical depth, essentially measures the amount of aerosols in the atmosphere (higher values mean higher sulfate aerosol concentrations). Both the values near the tropics (left) and averaged over most of the globe (right) show gradual increases in aerosol levels over the beginning of the 21st century. This is due to many smaller volcanic eruptions, after which large jumps in optical depth are seen for large known eruptions (depicted temporally by the vertical lines).
To quantitatively look at this correlation, the authors then plotted the correlation coefficient, r, between optical depth and sea surface temperature, troposphere temperature, water vapor, and precipitation. This graph is a little complicated to look at, but I’ll explain:The x-axis is again in years. The y-axis is the correlation coefficient r, which can take a value between -1 and 1. A value of 1 means that two variables increase and decrease together exactly, a perfect correlation. A value of -1 means they inversely relate exactly, one goes up in perfect synchrony with the other variable going down. Each colored line plots, across the years, the correlation between the amount of aerosols in the stratosphere and the relevant variable (all are listed below the figure).
A couple things to notice. First, here are large negative correlations in the time periods 1987-1996 and 2002-2013 between aerosol concentration and all the variables of interest – sea-surface temperature, water vapor, troposphere temperature, everything (precipitation to a lesser degree). These periods of time coincide with the years after major volcanic eruptions that are labeled in the figure – Pinatubo, Tavurvur, and Nabro. These correlations end up being statistically significant, and is a clear indicator that the eruptions are largely responsible for the decreases in sea surface temperature and lessened precipitation over these times. Correlation coefficients around 0.7, seen in the 1990’s, are huge! A coefficient that high means that the aerosol concentrations explain almost half of the variance in sea surface temperature, indicating that eruptions are one of the major factors in understanding the global warming hiatus.
So there it is – volcanic eruptions over the past couple decades have strongly influenced the concentration of sulfate aerosols in the stratosphere, leading to more reflected radiation and decreased sea surface temperatures. I should also note that all this data was analyzed after controlling for El Nino, which can have a large impact on seasonal changes in temperature and precipitation.
Anthropogenic global warming is still happening – natural variations in cycles will always suppress or enhance our influence on the climate. This paper strongly indicates that an upward flux in eruption frequency is suppressing warming trends that we are causing due to fossil fuel emissions. This means that, as the effects of these eruptions lessen, we will likely see a return to the warming trend in sea surface temperatures due to human emissions.
Santer, B., Solomon, S., Bonfils, C., Zelinka, M., Painter, J., Beltran, F., Fyfe, J., Johannesson, G., Mears, C., Ridley, D., Vernier, J., & Wentz, F. (2014). Observed multi-variable signals of late 20th and early 21st century volcanic activity Geophysical Research Letters DOI: 10.1002/2014GL062366