Extreme Weather

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Science tells us that extreme weather events, particularly heat waves, heavy precipitation, and drought, are increasing in frequency and severity as a direct result of climate change. The recent experience of extreme weather events around the globe matches the trends predicted by many climate scientists. Extreme weather is one of the key areas that allows the public to relate to and understand the concept and consequences of human-fueled climate change.

Heat waves are longer and hotter than they used to be and regions across the globe are suffering from catastrophic drought. Heavy rains are more frequent and can be more intense. Rainfall records have been washed away. These events fit a pattern that climate scientists have long expected to appear as the result of increased greenhouse gases in the Earth’s atmosphere. It is a fact that extreme weather was happening before global warming began, but the overwhelming scientific agreement is that global warming has contributed to a trend of more intense extremes of heat and precipitation around the world. There are various connections between increased carbon emissions in the atmosphere from human activity and a number of types of extreme weather including massive precipitation events, droughts, hurricanes, wildfires, flooding and heat waves.

Droughts

It remains scientifically challenging to say climate change caused a particular drought, however, drought is expected to increase in frequency, intensity, and duration in some regions as a result of human caused climate change. This is likely because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. The Palmer Drought Severity Index (PDSI) shows a decrease in moisture globally since the 1970s with a commensurate increase in the area of drought that is attributed, in part, to global warming. There are four areas directly attributable to climate change that contribute to the world’s drought problem. First, in a warming world, a larger fraction of total precipitation falls in downpours, meaning a larger fraction is lost to storm runoff instead of being absorbed in soil. Secondly, in mountain regions, a larger fraction of precipitation falls as rain rather than as snow, meaning lower stream flows in spring and summer. Snowpack is melting earlier in a warming world, further reducing flows later in the year. Last, where temperatures are higher, losses of water from soil and reservoirs due to evaporation are higher than they would otherwise be.

Flooding

Recent decades have brought more heavy summer rainfall events along with increased likelihood of devastating floods. Because warmer air can hold more moisture, global warming is expected to bring more and heavier precipitation in the years to come. In arid areas, which are more prevalent in a warming world, heavy rain in a few hours can produce flash flooding even in places where little rain has fallen for weeks or months. If heavy rainfall occurs repeatedly over a wide area, then river or mainstem flooding becomes more likely. When these conditions are in place, rivers of a region can swell and inundate large areas, sometimes well after rainfall has ended.

Beyond major precipitation events, flooding is a major concern worldwide because of sea level rise. Since 1900, sea level has gone up an average of eight inches around the world, due to global warming. By 2100, it is expected to be higher still — possibly as high as six-and-a-half feet above 1992 levels. Sea level rise is happening for two reasons. First, water expands as it warms. So the ocean is literally swelling and has nowhere to go but up. Second, more water is pouring into the sea as the ice on mountaintops and ice sheets in Greenland and Antarctica keep melting. The most recent reports from the Intergovernmental Panel on Climate Change, suggest that sea level rise will hit low lying areas like Bangladesh and southeast Asia particularly hard over the next 100 years. This destruction, along with devastation to major cities in the US and Europe, could displace millions and have untold impacts of health and economic concerns worldwide.

Heatwaves

The National Academy of Sciences reports that the hottest days are now hotter. And “the fingerprint of global warming behind this change has been firmly identified.” Since 1950 the number of heat waves worldwide have increased, and the length of heat waves has increased. Moreover, there has been in increase in extremely hot days. Put simply,the hottest days and nights have become hotter and more frequent. In the contiguous United States, new record high temperatures have consistently outnumbered new record lows by a ratio of 2:1 over the last ten years. In 2012, the ratio for the year through June 18 stands at more than 9:1.10 Worldwide heatwaves have been costly on several levels. Australia’s “Angry Summer” of  2012-2013 brought 150 temperature daily temperature records across the already hot nation. The Russian heat wave of 2010 was considered the nation’s worst heatwave in 1,000 years and was reportedly responsible for 15,000 deaths. . Historic heat wave in Texas in 2011 came in the middle of what is now several year long drought and 2012 saw the “Summer in March” in the US. Currently, a drought in California is the state’s longest and hottest and is a major threat to the state’s already threatened water supply.

Hurricanes, Typhoons and Tropical Cyclones

Tropical cyclones, called hurricanes in the Atlantic Ocean, typhoons in the Pacific and cyclones elsewhere are defined as closed atmospheric circulations with sustained wind speeds of at least 74 miles per hour. These storms are influenced by a number of factors, but three factors must be present for them to intensify: warm ocean temperatures, low vertical wind shear, and high humidity. As warm, moist air rises, it lowers air pressure at sea level and draws surrounding air inward and upward in a rotating pattern. The water vapor-laden air then spirals in and rises to higher altitudes, it then cools and releases heat as it condenses to rain. This cycle of evaporation and condensation brings the ocean’s heat energy into the vortex, powering the storm.

When it comes to climate change  fueling more intense hurricanes and typhoons, there are two factors that play a key role: ocean heat content and water vapor. Ocean heat content and water vapor have both increased over the past several decades. This is primarily due to human activities such as the burning of fossil fuels and the clearing of forests. Over the last 50 years, the world’s oceans have absorbed about 20 times as much heat as the atmosphere has.  This has lead to substantially warmer ocean temperatures at depths up to 1,500 feet deep. This warming causes oceans to expand and sea levels to rise. This warming also causes ice to melt in polar regions; this has already lead to global sea level rise of  more than one inch over the last decade. Over the last forty years, atmospheric humidity over oceans has increased 4 percent percent, which correlates with an increase in air temperature. These factors allow for tropical cyclones to become larger and more destructive. In recent years, many scientists have pointed to Hurricane Sandy in the United States and Typhoon Haiyan in the Philippines as examples of storms on steroids because of climate change.

Wildfires

Higher spring and summer temperatures, along with an earlier spring melt, are the primary factors driving the increasing frequency of large wildfires and lengthening the fire season in regions around the world.  2013 saw record-breaking fires in the American  Southwest and Rocky Mountain region. According to the U.S. National Academy of Sciences, climate change has likely contributed to a significant increase in big forest fires in the West. Over the past 30 years, large and long-duration forest fires in the American West have increased fourfold, the length of the fire season has expanded by 2.5 months, and the size of wildfires has increased several-fold. According to the Intergovernmental Panel on Climate Change’s most recent Working Group II report, “a moderate increase in precipitation, together with increased productivity, and a warming planet will likely generate more flammable fine fuels. An increase in risk, severity, and frequency of forest fires is expected to impact Europe. Similar predictions of an earlier start to the fire season and significant increases in the area experiencing high to extreme fire danger hold true in Canada and Russia.” The southern hemisphere is also not immune to climate factors affecting wildfires. “Fires along the edges of degraded peat-swamp forests in southern Sumatra and Kalimantan and similar fires in Brazil, show the importance of interaction between climate and human actions in determining the structure and composition of tropical forests, land-use patterns, and carbon emissions.


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  • “This is just the beginning. I’m not saying there’s going to be a Sandy every weekend or every year, but every decade we can expect more and more stronger storms.” Bill Nye the Science Guy
  • “Climate change amps up… basic factors that contribute to big storms… The oceans have warmed, providing more energy for storms, and the Earth’s atmosphere has warmed, so it retains more moisture which is drawn into storms, and is then dumped on us.” Mark Fischetti, Senior Editor of Scientific American
  • On extreme weather events: ‘It’s beginning to dawn on people that something is going on, that something bigger is afoot, the front line of climate change is right in the backyard, and it is also that it happens in ten other places at the same time’ – Susanne Moser, Research Fellow at the Woods Institute for the Environment at Stanford University, USA.
  • Dr. Andrew Steer of WRI: “The evidence is clear and mounting. The United States sits at the center of the climate crisis. Record heat is devastating crops, rivers are drying up, and storms are bearing down on our cities. Climate change is taking its toll on people and their economies, and will only become more intense without a strong and rapid response here in the United States and around the globe. It’s not too late to take action, but given lags in policy and geophysical processes, the window is closing.”

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