Climate Change or Weather

Last summer it was extremely hot and dry here in Prince William County, Virginia. It was the year my heat pump failed, others had wells go dry and the Interstate Commission on the Potomac River Basin (ICPRB) engaged a study for various climate scenarios of water supply availability from Potomac Watershed to determine if the water supply would be adequate to serve the population. This year is a different story. The summer has been cooler and wetter. Drought here is a distant memory. The summer is ending with only a couple of weeks this summer above 90 degrees and no days in triple digits and my garden is green.

The climate of the earth is constantly changing and the oceans rising for 10,000 years. Scientific studies and computer models have indicated that over the past century the earth has warmed 1.3°C. This warming is not particularly alarming in itself given our planetary history, but the speed of this temperature increase and the fact that the warming is projected to continue at an accelerated pace due to carbon dioxide concentrations in the atmosphere is worrisome. The planetary warming is forecast to cause sea levels to rise at an accelerated rate due to melting of sea ice in parts of the world, and changes in weather and patterns and precipitation. If carbon dioxide (CO2) concentrations in the atmosphere are the driving force in earth’s temperature the some portion of the weather extremes recently experienced are being caused by man.

According to the report “Explaining Extreme Events of 2012 from a Climate Perspective” released this week by the Bulletin of the American Meteorological Society, some of the extreme weather events of last year had mankind as one of the causes. Overall, 18 different research teams from around the world worked on the peer-reviewed report that examined the causes of 12 extreme weather events that occurred on five continents and in the Arctic during 2012. Hurricane Sandy slammed into the U.S. mid-Atlantic seaboard on October 29–30, 2012 causing widespread damage and devastating disruption to critical infrastructure. Hurricane Sandy broke 16 historical storm-tide levels along the East Coast though Sandy’s magnitude on the Saffir-Simpson hurricane wind scale was not particularly large; its westward strike heading was very unusual and it struck at high tide. Since 1851, nine other hurricanes (Category 1 and 2) have made landfall with similar proximity but all were heading north-northeastward. It was concluded that climate changes caused by man had no significant impact on that storm or the damage it caused . ($60.2 billion has been allocated by Congress to fund repair and mitigation measures.) However, the authors note that in the future rising sea levels could make smaller storms more likely to cause devastating damage.

Likewise, human-induced climate change was found to have had little impact on the lack of precipitation in the central United States in 2012 and continues in the current drought. However, in the section of the report titled, The Extreme March–May 2012 Warm Anomaly Over the Eastern United States by Thomas R. Knutson, Fanrong Zeng, and Andrew T. Wittenberg the authors found Approximately 35 %t of the extreme warmth experienced in the eastern U.S. between March and May 2012 can be attributed to human-induced climate change; and say high temperatures are now likely to occur four times as frequently due to human-induced climate change.

However the forecast is sensitive to the base period used and our assumptions of weather variability. The near-record Atlantic Ocean warmth off the east coast of the United States during March to May 2012 was annualized using a “multistep attribution” approach from Hegerl et al. (2009). This involves an assessment that attributes the observed change in seasonal mean temperature extremes to a change in climate and a separate assessment that attribute the change in climate and/or environmental conditions to external drivers and external factors. The observed trends in the figure below indicate that (according to the model-generated variability) the measured temperature extreme in 2012 were inconsistent with internal climate variability alone.

from Knutson et al

This was determined by using a control period of weather as a surrogate for the possible natural variability of temperatures. Since the heat wave of March-May 2012 occurred in a region with what the authors call “detectable long-term anthropogenic warming,” they concluded that anthropogenic forcing also likely contributed significantly to the observed temperatures in 2012. They state that a rough estimate of the anthropogenic contribution would be about 35% (based on the modeled value of ~1.3°C and the 2012 observed temperature anomaly of ~3.7°C). This 3.7°C event was 2.8 times stronger than the expected 1.3°C due to anthropogenic forcing in 2012. So, according to the authors weather variability played a substantial role.

The authors have simply assumed the 1.3°C portion of the anomaly is due to anthropogenic forcing as predicted by previous modeling of the climate. The estimate of the contribution of anthropogenic forcing to the observed weather variability are sensitive to two assumptions, the accuracy of 1.3°C increase in global temperatures numbers produced by models and the baseline period assumed by the authors. Here they used the period 1881–1920 as the baseline; if they used 1861–2012 as the baseline period, the risk of the event increases by about a factor of 5 rather than 12, and the portion of the temperature anomaly attributed to anthropogenic forcing would be 22%. If the average temperature increase were due to man were assumed to be lower, then the contribution of anthropogenic forcing would be less and vice versa.

The accuracy of climate models to regional variability is unclear. On a whole earth basis the climate models show at this point there is nothing that we can do to stop global warming and climate change. What is going to happen will happen.