Monday, November 12, 2012

The Rise Of Extreme Weather

Extreme weather has always been with us, but the trend over the past few decades has been for it to increase in number.  The Climate Extremes Index (CEI) was developed by the National Climatic Data Center (NCDC) and goes back to 1910.  It looks at the number of events that occur.  The graph below compares the period of January through October so that data from this year can be included.

The CEI for the period of January - October 1910 - 2012.  Image Credit: NOAA/NCDC.

The CEI with the experimental tropical cyclone indicator included.  Image Credit: NOAA/NCDC.

A new report recently released (before Hurricane Sandy) by Munich Re, the world’s largest reinsurance company, re-enforces this observation.  The report is about the increase in extreme weather over North America.  The report states:

Nowhere in the world is the rising number of natural catastrophes more evident than in North America. The study shows a nearly quintupled number of weather-related loss events in North America for the past three decades, compared with an increase factor of 4 in Asia, 2.5 in Africa, 2 in Europe and 1.5 in South America. Anthropogenic climate change is believed to contribute to this trend, though it influences various perils in different ways. Climate change particularly affects formation of heat-waves, droughts, intense precipitation events, and in the long run most probably also tropical cyclone intensity. The view that weather extremes are becoming more frequent and intense in various regions due to global warming is in keeping with current scientific findings, as set out in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) as well as in the special report on weather extremes and disasters (SREX). Up to now, however, the increasing losses caused by weather related natural catastrophes have been primarily driven by socio-economic factors, such as population growth, urban sprawl and increasing wealth.

Global warming heats the oceans thereby contributing to sea-level rise through thermal expansion.  It is leading to melting of glaciers worldwide further contributing to sea-level rise.  This enhances the storm surge when ocean storms make landfall.  There is more moisture in the air due to the warmer conditions.  This provides energy to storms when they form.

What is less certain is how this warming is affecting weather patterns.  Observations do show a change in the global circulation pattern of the earth.  Research suggests that melting Arctic sea ice is affecting weather patterns by helping to create blocking patterns in the atmosphere.  Thus weather patterns tend to get lock in place for extended periods.

Andrew Freedman wrote an article for Climate Central entitled “How Global Warming MadeHurricane Sandy Worse”.  The following appears in the article referring to the blocking pattern that was in effect during Sandy:

Some researchers who warn that climate change is already being felt in extreme weather events, such as Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo., are not yet convinced of the Arctic connection. Others, such as Hayhoe, think it is a “plausible theory” that is worth investigating, although she noted there is evidence that Arctic warming may cause more blocking during the winter rather than during the fall.

Stu Ostro, of the Weather Channel, has been investigating global warming and its effects on weather patterns since at least 2005.  He has observed that many of the extreme weather events occur in the presence of a blocking pattern.  Dr. Francis, of Rutgers University, has published research which indicates that the increase in blocking patterns is related to the warming of the Arctic, which is warming at twice the rate of the earth.  More research will have to be done to fully understand blocking patterns.

There has been much written about climate change and its effects on Hurricane/Superstorm Sandy.  Bloomberg Businessweek made headlines with its cover story “It’s Global Warming, Stupid”.  Another take by three atmospheric scientists was published on Politico’s website.

The cover of Bloomberg Businessweek.  Image Credit: Bloomberg Businessweek.

Often people ask me if global warming caused this storm or that storm.  It is understandable whenever an extreme weather event makes news.  However, again many confuse weather and climate.  A hurricane or any storm is a weather event.  Meanwhile, a climate event is often a long-term trend.  For example the 30-year moving average for annual precipitation in Columbia shows that the climate has been getting drier since the 1980s.  This does not look at one storm or one year.

However, storms do form against the background of climate.  If the climate changes, then the baseline changes.  What was once a normal year for precipitation in Columbia would now be considered a very wet year.  See my blog post entitled “Our Local Climate Change”.

Two directly related impacts of global warming on Sandy were sea-level rise and increased moisture in the atmosphere.  A one foot rise in water levels has been observed at Battery Park in lower Manhattan in the past century.  This helped push the storm surge to record levels.  In addition, global warming has resulted in an increase in atmospheric moisture of up to 7%.  The additional moisture adds energy to the storm which helps maintain and increase its intensity and structure.

My answer to the question of global warming effects is: Global warming doesn’t cause the storm; it causes the storm to be worse.  It doesn’t cause the heat wave; it causes the heat wave to be hotter.  It doesn’t cause the drought; it causes the drought to be drier.  It doesn’t cause the flood; it causes the flood to be higher.  In short, global warming doesn’t cause the weather you see; it causes the weather you see to be more extreme.  As Deke Arndt of NCDC has said “Climate trains the boxer, but weather throws the punches.”

Global warming will almost certainly continue for some time into the future, because of our addiction to burning fossil fuels for energy.  Therefore, it is very likely that the rise of extreme weather is only beginning.  Storms, heat waves, droughts, and floods will become more extreme as more energy becomes available in the atmosphere.

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