Extreme rain and snow events linked to global warming, study finds
Authors say a study that looked at the rise in extreme rain and snowfall in the Northern Hemisphere in the last half of the 20th century is the first to show a link to global warming.
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The team analyzed the maximum amount of precipitation each station recorded for a single day for each year as well as the maximum five-day amount of rain and snowfall.Skip to next paragraph
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They found that for that areas of the Northern Hemisphere the data covered, 65 percent of them saw increases in single-day precipitation extremes, while 61 percent saw increased in extremes during five-day events.
In addition, the team documents that on average the area covered by the study saw a 7 percent increase in the likelihood of experiencing an "average" extreme event in a given year.
"That's a substantial change over a 50-year time period," Dr. Zwiers says.
The team then used climate models to simulate 20th century climate to see if a link exists between global warming and the trends they saw in precipitation measurements.
They used model results of the climate's bahavior without the added CO2 and aerosols humans have pumped into the atmosphere. Then also ran simulations that included the additional CO2, as well as shifts in natural climate changers, such as shifts in solar radiation or aerosols from volcanic eruptions. When the team included these factors in their model runs, the simulated climate displayed the same general precipitation trends as the real-world measurements. But the models showed a far weaker effect than the real-world data showed.
For Isaac Held, a senior scientist with the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J., the gap between real world and model results should inject some caution into using these particular results as an initial signal that scientists have found a smoking gun where global warming and extreme events are concerned.
In some ways, he suggests, the study's main value may rest in the model shortcomings it may reveal – shortcomings "that may limit our current ability" to provide credible evidence of a link between global warming and the extreme rain and snowfall.
"Either something is wrong with the models and they're not getting the magnitude of the extremes observed," Dr. Held says, identifying global warming as a culprit "may be suspect because something is going on" in the climate system that the models aren't capturing.
Indeed, Zwiers acknowledges that this initial attempt at hunting for global warming's signature on extreme precipitation could benefit from updated observations as well as a new generation of improved climate models coming on line. Among the aspects of climate they aim to handle with greater sophistication: clouds and aerosols. These are two aspects of climate change whose effects on climate – and its future effects on them – carry some of the largest uncertainties when trying to project future effects from global warming.
These new models "may shed light on discrepancies between the magnitude of observed changes in extreme precipitation and that of simulated changes," Zwiers says.