August 4, 2012 — andyextance

Though the last time western North America saw a drought as severe as the one it experienced in 2000-2004 was 800 years ago, such conditions could become normal by the end of the century. This drying will shift western North America’s natural carbon cycle into reverse, from absorbing CO2 overall today to emitting it and worsening climate change further. That’s according to a paper by Christopher Schwalm, from Northern Arizona University in Flagstaff, and his colleagues published in the research journal Nature Geoscience on Sunday. “What we now call a drought event will become an abnormally wet episode by the end of the 21st Century,” Christopher told Simple Climate.

With drought in the US Midwest currently drawing much attention, the 2000-2004 drought might have slipped from some memories – even though they led to a natural disaster being declared in some areas. Our views of these events are bound to change as droughts become more regular as the world heats up, even though warmer air can contain more water vapour. “The changing climate means that the atmosphere can carry more water,” Christopher said. “However, precipitation will happen in spurts, more intensive rainfall in a relatively short period of time. Similarly, the amount of time between such events is expected to increase. So we will see fewer but more intense rainfall events, and therefore more droughts.”

The “turn of the century” drought has been remembered and closely studied by researchers looking at what it tells us about our climate. However, that research hasn’t been brought together to understand what it means for how much carbon plants are taking up, Christopher said. So he set out with a team of nine other US and Canada-based researchers to fill that gap. But to do so meant working with several sets of measurements that covered different areas and time periods. “Despite having various satellite data sources, global monitoring networks, and the like, our biggest challenge was how to piece together what data we do have and still paint a comprehensive picture,” Christopher explained.

Dragging together data

The scientists brought together information from monitoring networks ranging from river flows measured with ‘stream gauges’ and weather stations measuring soil moisture and rainfall to forest-based towers measuring exchanges of energy and CO2 between the air and earth. The satellite data they collected included net primary productivity – a measure of the speed of plant growth. Finally, they called on a record that reconstructs drought severity from tree-ring information back as far as 800 to see how unusual this drought was.

“Our approach was very straightforward,” Christopher said. “We contrasted data from the 2000-2004 drought with a neutral, drought-free, period 3 years before and after. Subtracting one from the other gives you information on the impact of the drought.” This comparison showed that crop productivity in much of the region fell 5 per cent, while runoff in the upper Colorado River basin was cut in half. The effect on CO2 absorption was equally dramatic. “The 2000-2004 drought severely decreased carbon uptake in the study area – halving it on average,” Christopher said. “What’s more, looking into the past a drought of this severity last occurred over 800 years ago.”

To understand how common such droughts will be in future, the researchers turned to a group of climate models, assuming CO2 emissions continue to increase. They predict the first “megadrought” of modern times, and that the ability of the area studied to absorb carbon will “collapse”. “Looking into the future droughts of similar severity will become commonplace,” Christopher said. “Also, a whole host of significant water resource challenges, especially difficult for a region already subject to frequent water shortages, would result.”

Christopher and his colleagues are now planning to investigate the current Midwest drought in the same way. Though they have both occurred in a warming world, where scientists predict longer, more severe droughts, it is otherwise hard to compare them. “The affected areas do not overlap,” the scientist said. “However, we expect that a similar study on the current drought would have similar conclusions. While it is difficult to attribute any one event to changing climate, the warming of the planet attributed to human activities is expected to make any such events more severe.”