A coalition of 47 environment, science, and faith-based groups have sent a letter to President Barack Obama asking him to pursue a goal of keeping global warming to less than 2 degrees Celcius in upcoming international meetings.

“Failure to limit warming to 2 degrees Celsius will have the greatest impact on the most vulnerable nations and communities and will dramatically increase the need for adaptation in the future,” they write.

The groups include those on the left side of the green spectrum like Greenpeace and Friends of the Earth, as well as more moderate groups like the Environmental Defense Fund and Natural Resources Defense Council. The letter represents a notable show of unity between groups currently at odds over climate legislation — Greenpeace and FOE opposed the bill that passed the House last week, arguing that it is too weak, while EDF and NRDC were key players in brokering the deal.

The organizations call on Obama to endorse the 2-degree threshold next month in Italy at the G8 Summit and the Major Economies Forum on Climate and Energy, to set the playing field for the global climate negotiations in Copenhagen in December. The groups note that 124 countries have agreed on the 2-degree limit, and that the goal is endorsed in the House climate bill.

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Here’s the letter, and the full list of signatories:

June 26, 2009

Dear President Obama,

We are writing to urge you to work with other world leaders at the upcoming G8 Summit to set a strong science-based goal for reductions in emissions of greenhouse gases aimed at keeping the increase of global average surface temperature, compared to pre-industrial levels, as far below 2 degrees Celsius/3.6 degrees Fahrenheit as possible.

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Global warming will have severe (in many cases catastrophic) impacts on populations and countries throughout the world, including the US. The United States, along with 192 other nations, has committed itself to the ultimate objective of the United Nations Framework Convention on Climate Change: achieving stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Failure to limit warming to 2 degrees Celsius will have the greatest impact on the most vulnerable nations and communities and will dramatically increase the need for adaptation in the future.

The recent U.S. Global Change Research Program report makes it clear that there is no escaping global warming. Heat waves will become more frequent and intense; increased heavy downpours will lead to more flooding, waterborne diseases, and negative effects on agriculture; rising water temperatures and ocean acidification will threaten coral reefs and the rich ecosystems they support; and local sea-level rise of over three feet on top of storm surges will increasingly threaten homes and other coastal infrastructure. The report also summarizes the latest climate change scenarios and concludes, Resulting temperature changes depend on atmospheric concentrations of greenhouse gases and particles and the climate’s sensitivity to those concentrations … only the 450 ppm stabilization target has the potential to keep the global temperature rise at or below about 3.5°F from pre-industrial levels and 2°F above the current average temperature, a level beyond which many concerns have been raised about dangerous human interference with the climate system. Scenarios that stabilize carbon dioxide below 450 ppm offer an increased chance of avoiding dangerous climate change.”

Our landmark U.S. environmental laws are founded on the best science and the pragmatic goal of designing policies that are capable of solving the problem. The goal of the Clean Water Act is for all waters to be fishable and swimmable. The goal of the Clean Air Act is to promote the public health and welfare and requires our regulations to “accurately reflect the latest scientific knowledge.” Our domestic global warming policies, as well as the international agreements we sign, should be aimed at protecting our population and broader national interests, as well as protecting the world’s most vulnerable communities from the worst impacts of climate change. Staying below the 2°C/3.6°F target is a key guide post to measure our efforts to tackle global warming pollution. While the laws and regulations on the books may not always achieve these goals as quickly or as completely as we might hope, this guide post will keep us vigilant in reviewing and renewing our efforts.

Some 124 countries, along with scientists and citizen groups here in the United States and around the world, have called for holding global temperature increase below 2°C. Staying below 2°C is the stated goal of the American Clean Energy and Security Act that passed out of the House Energy and Commerce Committee, as well as the Global Warming Pollution Reduction Act of 2007, which you co-sponsored as a Senator. This goal was also the basis for the climate and energy recommendations to your transition team from the 29 largest U.S. environmental, conservation, and development organizations.

The G8 Summit and the Major Economies Forum on Climate and Energy in Italy next month will provide the opportunity to forge an important consensus among key nations toward a new global climate agreement in Copenhagen. The statements emerging from those meetings must reassure citizens in the United States and around the world that our leaders understand the threat that is before us and are courageously taking the steps necessary to protect us. Here at home, you have committed to develop and implement a comprehensive clean energy plan that will generate jobs, reduce dependence on oil, and reduce carbon pollution. By stating your commitment to keeping global average temperature rise compared to pre-industrial levels below 2°C/3.6°F, and working with other leaders at the G8 and MEF to affirm this goal, you will take an important step towards making sure the global climate agreement in Copenhagen this December helps achieve your goal of protecting Americans, and the rest of the world, from the worst potential effects of climate change.

Groups signing the letter:

1 Sky
ActionAid USA
Center for International Environmental Law
Center for Biological Diversity
Chesapeake Climate Action Network
Clean Water Action
Climate Action Network International
Climate Solutions
Climate Law & Policy Project
Conservation International
Defenders of Wildlife
Education for Global Warming Solutions
Environmental and Energy Study Institute
Energy Action
Environment Northeast
Environment America
Environmental Defense Fund
Environmental Law & Policy Center
Fresh Energy
Friends of the Earth
Green For All
Institute for Policy Studies
Interfaith Power and Light
International Forum on Globalization
International Rivers
League of Conservation Voters
National Audubon Society
Natural Resources Defense Council
National Wildlife Federation
Oil Change International
Oxfam America
Physicians for Social Responsibility
Pew Environment Group
Sierra Club
Southern Alliance for Clean Energy
Sustainable Obtainable Solutions
The Nature Conservancy
Union of Concerned Scientists
US Climate Action Network
World Wildlife Fund