Fire and water – how global warming is making weather more extreme and costing us money

Trees burn as flames move towards the City of Berkeley's Toulumne Family Camp near Groveland, California in August 2013. Global warming creates conditions that intensify wildfires and the costs of fighting them.
Trees burn as flames move towards the City of Berkeley’s Toulumne Family Camp near Groveland, California in August 2013. Global warming creates conditions that intensify wildfires and the costs of fighting them. Photograph: Noah Berger/EPA

Connecting the dots between human-caused global warming and specific extreme weather events has been a challenge for climate scientists, but recent research has made significant advances in this area. Links have been found between some very damaging extreme weather events and climate change.

For example, research has shown that a “dipole” has formed in the atmosphere over North America, with a high pressure ridge off the west coast, and a low pressure trough over the central and eastern portion of the continent.

Departure of the November 2013 – January 2014 250 hPa geopotential height from the normal climatology.
Departure of the November 2013 – January 2014 250 hPa geopotential height from the normal climatology. Source: Wang et al. (2014), Geophysical Research LettersPhotograph: Wang et al. (2014), Geophysical Research Letters

These sorts of pressure ridges in the atmosphere are linked to “waves” in the jet stream. Research has shown that when these jet stream waves form, they’re accompanied by more intense extreme weather. The high pressure zone off the west coast or North America has been termed the “Ridiculously Resilient Ridge” due to its persistence over the past two years. It’s been the main cause of California’s intense drought by pushing rain storms around the state.

California drought as of 26 August 2014.  58% of the state is in 'exceptional drought' conditions.
California drought as of 26 August 2014. 58% of the state is in ‘exceptional drought’ conditions. Source: United States Drought Monitor

A paper led by S.-Y. Wang of Utah State University found the high pressure ridge is linked to a precursor of the El Niño Southern Oscillation (ENSO), but also that human-caused global warming has amplified the strength of these ridges. The authors concluded,

It is important to note that the dipole is projected to intensify, which implies that the periodic and inevitable droughts California will experience will exhibit more severity.

Similarly, a recent paper led by Kevin Trenberth and published in Nature Climate Change concluded,

Increased heating from global warming may not cause droughts but it is expected that when droughts occur they are likely to set in quicker and be more intense.

Another study recently published in the Journal of Climate examined data from past climate changes, and found that climate models are underestimating the likelihood of intense droughts in the southwestern USA due to global warming.

In the US Southwest, for instance, state-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%; our analysis suggests that the risk is at least 80%, and may be higher than 90% in certain areas. The likelihood of longer lived events (> 35 years) is between 20% and 50%, and the risk of an unprecedented 50 year megadrought is non-negligible under the most severe warming scenario (5-10%).

There are several ways in which global warming intensifies drought. Hotter temperatures increase evaporation from soil and reservoirs. They cause more precipitation to fall as rain and less as snow, which for a region like California that relies on the snowpack in the Sierra Nevada mountains as its natural water storage system, is problematic. Hotter temperatures also cause the snowpack to melt earlier in the year. The problem can be alleviated by building more water storage infrastructure, but that costs money.

On top of all that, there’s the apparent strengthening of high pressure ridges off the coast, pushing rain storms around California. Research suggests that there may be a connection between these ridges and the decline in Arctic sea ice, although this connection is debated among climate experts.

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