by Kevin E. Trenberth Wednesday, November 7, 2012
This story was written in September and published in our December "Highlights of 2012" issue, which came out in November. Update below.
In summer, there is typically a strong inverse relationship between rainfall and temperature. If it is wet, it tends to be cool; if it is dry it tends to be hot. During the Dust Bowl era of the 1930s, the air over much of the U.S. was incredibly dry, which led to many record high temperatures. Most of those temperatures have been eclipsed in recent years except for the July 1936 average temperature record. Now that too has been exceeded.
July 2012 was the hottest month by far for the lower 48 states. Much of the nation faced drought conditions that grew steadily worse throughout the summer, and there were major repercussions for crop yields and food prices. Wildfires were also rampant. The record low snowpack in May 2012 in the Colorado Rockies set the stage for major wildfires in June, with more than 600 homes lost in Colorado alone. Wildfires developed in other regions in July as well, as tremendous record-breaking heat developed in Oklahoma and surrounding areas.
Considered individually, the record temperatures, droughts, fires and diminished snowpack are not necessarily alarming and may not signal anything beyond the natural occurrence of a hotter-than-average year. But combined, the fact that the first seven months of 2012 were the hottest on record, more than half of U.S. counties were declared disaster areas due to drought, and that snowpack was at all-time lows, these indicators are much more significant from a climate standpoint. They highlight that there is more than just natural variability playing a role: Global warming has reared its head in a way that can only be a major warning for the future. So, what can we expect?
Both drought and extreme rain events are more intense these days. Moreover, the odds of all these extremes occurring have increased. Where these disasters occur is largely determined by natural patterns of weather, perhaps related to changes in ocean temperatures such as the El Niño phenomenon. But the intensity of the resulting droughts in some areas, and floods in others, is growing and records are being broken all over the place. This is to be expected with a warming climate.
To answer the inevitable question, “Will we see the same thing in 2013?": No, it’s unlikely the U.S. will see exactly the same situation next year. But these conditions will likely occur somewhere.
Drought and heat in one region is often matched by very wet conditions and even flooding in other areas. For meteorologists, this phenomenon is not surprising owing to atmospheric waves that cause high pressure (anticyclones) and fine weather in some spots, but low pressure storms (cyclones) and unsettled weather in other regions. In 2009, the hot spot was southern Australia around Melbourne, where the heat wave was one of the most extreme in the region’s history and was responsible for the deadly Black Saturday fires; meanwhile severe flooding struck the U.S. northern plains. In 2010, Russia was scorching, with the most extreme heat wave in the instrumental record for that region killing thousands of people; in the meantime, flooding hit Pakistan, Australia and Colombia. In 2011, Texas was exceptionally hot and dry, setting countless new high temperature records, and costing billions in drought- and heat-related crop losses; meanwhile, the Missouri and Mississippi rivers flooded and very wet conditions also struck the Northeastern U.S. And in 2012, much of the coterminous United States was beset by record-high temperatures and drought — also costing billions in drought- and heat-related crop losses, surpassing 2011’s losses by August — while the U.K. experienced a record wet spring and severe flooding struck southern Russia, Japan, North Korea and Beijing, China. Where will these varying conditions occur in 2013? No one knows yet.
The first part of 2012 was marked by La Niña conditions in the tropical Pacific. The development of a weak El Niño warming event in the second half of 2012 changed the situation, however, producing a weak Atlantic hurricane season* as tropical storm activity shifted to the Pacific arena. The shift brought a welcome change in weather patterns across the U.S., such as cooler and wetter weather to the South. However, the shift also has negative impacts elsewhere, like in Australia where it changes from flood-prone to drought-prone. All too soon, the plants that bloomed in wet conditions provide fuel for wildfires.
Increases in extreme climate events, such as droughts, heavy rains, floods, heat waves and wildfires, are usually bad for everyone. Water resources are increasingly stretched as demand increases, and food resources dwindle as drought, heat or floods knock out crops. Climate change has a cost — billions of dollars in 2012 in the U.S. alone.
Some costs are of course incurred every year due to weather, but many of the biggest losses due to extreme and record conditions likely would not have occurred without the human-induced component of climate change. Perhaps many of the fires would not have exploded out of control. These costs are often overlooked when considering efforts to address the causes of climate change.
Looking ahead, we as a nation need to face up to climate change issues, plan adequately for their consequences and work to stop the problem from becoming worse.
Climate change is a global problem, but the United States must show international leadership — from both moral and ethical standpoints — because we are historically responsible for most of the increase in heat-trapping greenhouse gases. Without such leadership, progress will be limited. Much can be done. Most important is recognition that there is a cost to climate change not reflected in the price of fossil fuels, and hence a carbon “tax” of some kind is much needed. And owing to the long lifetime of carbon dioxide in the atmosphere, the time to begin is now.
[Update, Nov. 14:]
Guessing in September what the rest of the year will be like does not always work. The Atlantic hurricane season was obviously dominated by Superstorm Sandy in the last week of October. Sandy started as an ordinary hurricane, feeding on the warm surface waters of the Atlantic Ocean for fuel. The warm moist air spiraled into the storm, and as moisture rained out, it provided the heat needed to drive the storm clouds. By the time Sandy made landfall, it had become an extratropical cyclone with some tropical storm characteristics: a lot of active thunderstorms but no eye.
This transformation came about as a winter storm that had dumped snow in Colorado late the previous week merged with Sandy to form a hybrid storm that was also able to feed on the mid-latitude temperature contrasts. The resulting storm — double the size of a normal hurricane — spread hurricane-force winds over a huge area of the United States as it made landfall.
Meanwhile an extensive easterly wind fetch had already resulted in piled up sea waters along the Atlantic coast. This, in addition to the high tide, a favorable moon phase, and exceedingly low pressure, brought a record-setting storm surge that reached four meters in lower Manhattan and coastal New Jersey. This "perfect storm" combination led to coastal erosion, massive flooding and extensive wind damage that caused billions in dollars of damage. The human influence on this storm came through the high ocean temperatures and associated heavy rains,and high sea level.
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