Why Has 2010 Been So Warm?
January though June, 2010 has been the warmest January-June period since 1880 according NASA and NOAA (see my last post). And the first decade of the 21st century (2000-2009) has been the warmest decade since 1880. Clearly, if you’re a betting person, you’d bet on continued global warming. As I noted in my last post, I did bet that 2010 would be a record breaker. However, being a climate scientist, I based my bid for a nice bottle of wine on more than just the global warming trend. After all, there are many reasons a year, two or even more can be colder than average even as the globe warms up due to human-caused emissions of greenhouse gases into the atmosphere.
My focus was less on trend, and more on causation—I asked myself whether all the known causes of climate change were going in the right direction for a record-breaking 2010.
The big causal agent, of course, is the atmospheric concentration of heat-trapping greenhouse gases (e.g., carbon dioxide, methane, nitrous oxide and others). And, despite the global recession, these concentrations keep going up.
Next, is there a big volcanic eruption that could derail my quest for wine? Here, I gambled some because it is impossible to predict a big climate-altering volcanic eruption—that is, an eruption that puts lots of material (e.g., sulfate) in the upper atmosphere (stratosphere) where it can reflect incoming solar radiation back to space and cool the globe for a few years before the volcanic material falls back to Earth. Many will remember well the eruptions of Eyjafjallajökull in Iceland starting in April, 2010. However, as disruptive as these eruptions were for air travelers, they did not move material into the stratosphere and impact climate. I’ll have to keep my fingers crossed that we don’t get another Mount Pinatubo-like eruption and the associated global cooling that we saw after 1991. Or like the 1982 eruption of El Chichón, which canceled out some of the global warming associated with the big El Niño of 1982-83.
Ah, El Niño… this was the ace in my deck. I knew that an El Niño, even a relatively moderate one such as just ended, can contribute to above normal global temperatures. Why? Because an El Niño warms enough of the tropical Pacific Ocean to have a small, but sometimes significant impact on the average temperature of the globe. For example, 1998 was unusually warm likely due, in part, to the big El Niño of 1997-98 (and no big volcanic eruption!).
What about the Sun? We know the Sun’s output is not constant, and that it can influence climate over decades and longer. But over the past century, its impact on temperature has been tiny compared to the influence of rapidly rising concentrations of atmospheric greenhouse gases (as described in the latest report of the Intergovernmental Panel on Climate Change). Even during this “no-sunspot” year, the impact on temperature is not likely to be large. Bottom line on solar: not a factor to worry much about for 2010 or any other year.
In future posts, we can talk more about climate change and effect, but here we’re only talking about the unusually warm first decade of the 21st century, and why I bet that 2010 might turn out to be the warmest year since 1880. And since the biggest influence—by far—on climate change for the coming century and beyond will continue to be atmospheric greenhouse gas increases, I’m betting on continued warming until human greenhouse gas emissions are reduced dramatically. And don’t let a cool year or two fool you—for those of us in the Southwest, this means continued warming and more. I’ll write about this in my next post.