Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=224921
Story Retrieval Date: 4/17/2015 11:35:01 AM CST
Wallace Broecker/ COLUMBIA UNIVERSITY
Global warming show a continuous rise since 1975, but critics of climate change recently pointed to a period of apparent stability in global temperatures spanning the last 15 years.
At this week's Comer Conference on Abrupt Climate Change, scientists countered that oceans are temporarily absorbing heat and masking global warming. They showed how patterns from the last 200 years predict future temperature rise.
The temperature rise, forced by human use of fossil fuels, is linked to increasingly extreme weather, drought and sea level rise that threatens coastal communities. It’s the rising oceans that are providing respite from rising global temperatures, scientists said.
Scientists have mapped how natural cycles in global ocean circulation play an important role in transporting and storing heat, including the extra energy arising from the burning of fossil fuels that release carbon dioxide. The greenhouse gas gathers in the atmosphere with every bit of fossil fuel we burn.
Both the Atlantic and Pacific oceans experience a variability in surface temperatures, from relatively cool to relatively warm every 60 to 80 years. Scientists have measured sea surface temperatures as far back as the mid- 1800s. Sean Birkel, a climate modeler at the Climate Change Institute at the University of Maine, said this variability plays a role in the temperature of Earth’s surface today.
“Since the mid-1990s, North Atlantic surface temperatures have registered in the warm phase of known natural variability. But it is important to note that this pattern of oscillating sea-surface temperature has an upward trend," likely due to increasing CO2 levels in the atmosphere, he said at the conference. CO2, a greenhouse gas that holds heat in the atmosphere, is increasing every year with the use of fossil fuels.
As global warming and ocean acidification continue, the world’s oceans may start to absorb less CO2, which would mean even more of this greenhouse gas would accumulate in the atmosphere. Absorbed carbon dioxide could also come back into the atmosphere if it is part of an ocean warming cycle like El Niño or an ocean cooling cycle like La Niña.
“Where we’re going, it doesn’t [matter] if you put the heat in the ocean first or in the atmosphere first. Eventually, they both get warm and we wind up in the same place,” said Richard Alley, a geoscientist at Pennsylvania State University.
“The world is getting warmer, with very high confidence. Someone who doesn’t wish to confront this can always shut up during the new record and then yell that global warming stopped again,” said Alley.
And that warming is anything but short term, scientists said.
“Half of the radiative forcing from each gigaton of CO2 you emit," stays in the atmophere for at least a thousand years," said Raymond Pierrehumbert, professor of geophysical sciences at the University of Chicago.
Pierrehumbert’s research confirmed a consensus among climate scientists that regardless of natural variations in ocean circulation and other factors that may stall the global temperature record, CO2 emissions are central to a warming climate. Alley nodded to this conclusion.
The 2013 Intergovernmental Panel on Climate Change draft summary for policymakers projects a likely 2 degree Celsius (more than 3.5 degrees Fahrenheit) increase in global temperatures resulting from the doubling of CO2 over the next 70 years.
The Comer Conference on Abrupt Climate Change draws top scientists to the Comer Foundation retreat in Southwestern Wisconsin each year.