Climate change already having visible impacts across US, and temperatures could rise another 2.2C in coming decades
By Gerard Wynn
Climate change is having widespread, visible impacts on the US people and economy, a government report concluded on Tuesday.
The National Climate Assessment was the third and most recent in a series published by the United States every four years and mandated by the US government in 1990.
The latest assessment found climate impacts were now felt across the country, and reported the strongest evidence yet of the human contribution to warming.
“Climate change, once considered an issue for a distant future, has moved firmly into the present,” it said.
“This National Climate Assessment concludes that the evidence of human-induced climate change continues to strengthen and that impacts are increasing across the country. Climate change presents a major challenge for society.”
US average temperature had risen by 0.7C to 1.1C since 1895 , and most of this increase has occurred since 1970, with temperatures rising fastest in the far north.
The report said temperatures would rise another 1.1C to 2.2C in coming decades.
“The most recent decade was the nation’s and the world’s hottest on record, and 2012 was the hottest year on record in the continental United States,” it said.
The report warned that a recent slowdown in global average warming should not deflect attention from the threat.
“Because human-induced warming is superimposed on a background of natural variations in climate, warming is not uniform over time. A recent slowing in the rate of surface air temperature rise appears to be related to cyclic changes in the oceans and in the sun’s energy output, as well as a series of small volcanic eruptions and other factors.”
“Nonetheless, global temperatures are still on the rise and are expected to rise further.”
The report found particularly high stakes regarding sea level rise, because nearly five million Americans and hundreds of billions of dollars of property were located in areas less than four feet above the local high-tide level.
The report said that sea levels were projected to rise by 0.3 to 1.2 metres this century.
The assessment reports are compiled by the research arms of various federal US departments and agencies, including the departments of agriculture, defence and energy.
The US Assessment agreed with the recent UN climate panel report, in its main conclusions that human activities were the main cause of warming since the mid twentieth century; that climate impacts were already visible; and that the world should slash carbon emissions to avoid the worst effects.
“Precipitation patterns are changing, sea level is rising, the oceans are becoming more acidic, and the frequency and intensity of some extreme weather events are increasing. Many lines of independent evidence demonstrate that the rapid warming of the past half-century is due primarily to human activities,” the US report said.
While it was difficult to attribute individual extreme events to climate change, the report said that the types of event associated with rising greenhouse emissions such as floods, droughts and extreme heat, were becoming more frequent or intense.
“Evidence indicates that the human influence on climate has already roughly doubled the probability of extreme heat events such as the record-breaking summer heat experienced in 2011 in Texas and Oklahoma.”
In other evidence for climate change, the report found rising sea levels; melting glaciers and Arctic sea ice; changes in natural systems including species distribution for example in response to changing seasons; and changes in seasonal allergies.
Regarding impacts, sectors especially affected by climate changes included agriculture, water, human health, energy, transportation, forests, and ecosystems.
Climate changes were in some cases already having extreme impacts, such as more regular flooding during storms and high tides in cities near coasts and large rivers; more severe wildfires in western states; and more coastal erosion in Alaskan towns previously protected by sea ice in winter.
There were fewer positive impacts, and mostly in the short run only, such as a longer growing season in some areas, and an extended shipping season in the Great Lakes.
“Many more are detrimental, largely because our society and its infrastructure were designed for the climate that we have had, not the rapidly changing climate we now have and can expect in the future.”
The US report said that the science of climate change was more solid than ever.
“Increased scrutiny has led to increased certainty that we are now seeing impacts associated with human-induced climate change. With each passing year, the accumulating evidence further expands our understanding and extends the record of observed trends.”
“It is notable that as these data records have grown longer and climate models have become more comprehensive, earlier predictions have largely been confirmed. The only real surprises have been that some changes, such as sea level rise and Arctic sea ice decline, have outpaced earlier projections.”
The report confirmed that human activities were the main cause for warming observed since the middle of the last century, ruling out the influence of other factors.
“Data show that natural factors like the sun and volcanoes cannot have caused the warming observed over the past 50 years.”
“In fact, if not for human activities, global climate would actually have cooled slightly over the past 50 years,” it said, for example owing to the cooling influence of major volcanic eruptions such as Mount Pinatubo in the Philippines.