An oft-referred to global warming projection is an increase in the frequency and intensity of droughts. But averaged over the globe, a new study published in Nature Wednesday finds little change in drought over the past 60 years.
Just a few years ago the prevailing wisdom was that a global drying trend had commenced in the 1970s. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC)concluded: “More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation has contributed to changes in drought.”
But the IPCC backtracked some from that statement in a 2011 report on climate change and extreme events. The report simply stated there is medium confidence some parts of the world have experienced more intense and longer droughts while other regions have seen decreases in drought, including central North America.
The results of this new Nature study seem to affirm that a clear global drought signal - that one could link to global warming - has not yet emerged.
“Here we show that the previously reported increase in global drought is overestimated,” the study authored by Princeton researchers Justin Sheffield and Eric Wood and Australian researcher Michael Roderick says. “More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years.”
The study’s findings have been challenged by at least one scientist: Aiguo Dai, an atmospheric scientist at the State University of New York at Albany. Dai wrote a 2011 review paper that found: “Global aridity has increased substantially since the 1970s due to recent drying over Africa, southern Europe, East and South Asia, and eastern Australia.”
Dai told Science News the new Nature study does not take into account trends in soil moisture and other variables and relies on some outdated and questionable data.
Methodological questions about the study notwithstanding, what’s clear is that identification of global trends in drought is “not so cut-and-dried”. Further, the study reinforces the point that exactly how a warming climate will influence precipitation patterns is complicated.
“The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes,” the study says.
How drought changes in a warming climate is a crucial issue. Just today, wunderground meteorologist Jeff Masters wrote a provocative blog post titled “Lessons from 2012: Droughts, not Hurricanes, are the Greater Danger”. Masters writes:
Drought is our greatest enemy. Drought impacts the two things we need to live--food and water. The history of civilization is filled with tales of great storms that have killed thousands and caused untold suffering and destruction. But cities impacted by great storms inevitably recover and rebuild, often stronger than before. I expect that New York City, the coast of New Jersey, and other areas battered by Sandy will do likewise. But drought can crash civilizations.
We should not assume that the 21st century global civilization is immune from collapse due to drought. If we continue on our current path of ever-increasing emissions of carbon dioxide, the hotter planet that we will create will surely spawn droughts far more intense than any seen in recorded history, severely testing the ability of our highly interconnected global economy to cope.
I can see two reactions to Masters’ commentary:
1) He’s prudently sounding the alarm. Global warming increases the risk of drought through higher temperatures and increased evaporation. The kind of drought the U.S. is experiencing in 2012 is a dangerous sign of things to come in the U.S. and globally.
2) Masters’ commentary is over the top especially in light of this new Nature study showing no increase in global drought. How precipitation will change in the future is complicated and we should not assume the worst.
What do you think?