Warming is well under way in US, scientists say
A warming US climate is already manifesting itself in rising tides, longer summers with more prolonged heat waves, heavier downpours, increasing wildfires, and changes in varieties of plant and bird species, according to a report by the US Global Change Research Program (USGCRP), a 13-agency collaboration involving nearly 300 scientists.
“Observations unequivocally show that climate is changing and that the warming of the past 50 years is primarily due to human-induced emissions of heat-trapping gases,” says the report, known as the National Climate Assessment. Trends in temperature, precipitation, sea level, ice mass, and many other variables recorded and analyzed by independent research groups from around the world have largely confirmed earlier predictions of climate change. “The only real surprises have been that some changes, such as sea level rise and Arctic sea decline, have outpaced earlier projections.”
Most of the US average temperature increase of up to 1.1 °C since 1895 has occurred since 1970, and the 2000–2010 decade was the nation’s and the world’s hottest on record. The report, developed over four years, projects a temperature rise of 1.1–2.2 °C in most areas of the US over the next few decades. If emissions from fossil-fuel combustion are left unchecked, temperatures could increase by as much as 5.6 °C by the end of the century. Even with substantial reductions in carbon dioxide emissions, the temperature could rise as much as 2.8 °C by 2100, says the report, which is based on the results from 16 climate models.
The US assessment mirrors the recently released reports from the Intergovernmental Panel on Climate Change, which warns that without any further policy actions to control emissions of greenhouse gases, the world is likely to see a rise in the average global temperature of up to 4.8 °C by the end of the century. That is more than double the 2° from preindustrial-era temperatures that scientists have estimated to be the upper limit if the worst impacts of climate change are to be averted, according to the panel.
For the contiguous 48 states, seven of the warmest years on record have occurred since 1998, and 2012 was the hottest year ever recorded. Climate and weather disasters in 2012, including Hurricane Sandy, drought, heat waves, and western wildfires, cost the US economy more than $100 billion, the USGCRP report says.
Since 1992 the rate of global sea-level rise measured by satellites has accelerated to roughly twice the rate observed over the past century. As the oceans expand along with rising temperatures and the melting of land-based ice, sea level is projected to rise another 0.3–1.2 meters this century, according to the report. Nearly 5 million Americans and hundreds of billions of dollars worth of property are located in areas that are less than 1.2 meters above the high-tide level. Ocean surface waters have become 30% more acidic in the past 250 years as they have absorbed large amounts of atmospheric CO2, which has reduced the ability of shellfish and corals to form their shells.
The impacts of a warming climate vary by region, from increasing heat waves, extreme precipitation events, and sea-level rise in the Northeast to drought, longer wildfire seasons, and growing water scarcity in the Southwest. In the Midwest, the benefits of a longer growing season have been offset by the increasing frequency of extreme weather events.
“This assessment is the most comprehensive and authoritative account ever about how climate has been changing in the United States, how it’s projected to continue to change in the future, and what can be done by public officials, planners, businesses, and individuals to reduce its impacts,” John Holdren, President Obama’s science adviser, said in a statement. “The contents confirm that climate change is not a distant threat. It is affecting the American people already.”
John Podesta, a senior adviser to Obama, told reporters that building resilience to sea-level rise, especially along the eastern seaboard and the Gulf of Mexico, and coping with the increasing incidence of wildfires are two particularly urgent responses the US should take.
According to the assessment, if only natural processes were influencing the climate, global temperatures would have stabilized at approximately 1980 levels. Only the addition of anthropogenic forces can explain the observed temperature increases that have occurred over that time, it said.
In recent years the US has been the second largest emitter of greenhouse gases, contributing about 18% of the world total. China is the world’s top emitter.