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Climate scientists: “The urgent need to act cannot be overstated.”

“Climate change caused by humans is already affecting our lives and livelihoods “” with extreme storms, unusual floods and droughts, intense heat waves, rising seas and many changes in biological systems “” as climate scientists have projected.”

Today, a large body of evidence has been collected to support the broad scientific understanding that global climate warming, as evident these last few decades, is unprecedented for the past 1000 years “” and this change is due to human activities. This conclusion is based on decades of rigorous research by thousands of scientists and endorsed by all of the world’s major national science academies….

Although uncertainties remain, they concern issues like the rate of melting of major ice sheets rather than the broader topic of whether the climate is changing.

This is from an article in the Politico, “The science behind climate science,” by four leading climate scientists: Dr. James McCarthy, Dr. Lisa Graumlich, Dr. Chris Field, and Dr. James Hurrell. You may remember Dr. Field from his terrific talk at CAP earlier this year.

Here’s more of the piece:

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How we as a society use what we have learned from climate science could define our generation.

Right now, our nation “” and the world “” are at a crossroads. Yet we seem stalled “” despite an increasingly clear picture of what human-induced global warming is doing to our planet.

The scientific community “” often working closely with governments “” has produced numerous, carefully reviewed, international and national assessments of the scientific understanding behind climate change. The latest, “America’s Climate Choices,” recently released by the U.S. National Academy of Sciences, states that “scientific evidence that the Earth is warming is now overwhelming.”

For more on the NAS report, see U.S. National Academy of Sciences labels as “settled facts” that “the Earth system is warming and that much of this warming is very likely due to human activities.”

So it is imperative that the public and policymakers question and debate the range of options for addressing climate change “” including the level of certainty about that evidence. But a fair debate requires understanding of the scientific process….

The path from theory to understanding is often far from simple. Sometimes, new insights lead to dramatic shifts in prevailing scientific views.

Consider the identification of the ozone hole in the 1980s. A consensus emerged among experts within a few years of finding key evidence “” though a small number of experts remained unconvinced.

Such is the case with climate science. Theories and observations have been tested, retested and reviewed.

Then come the key quotes excerpted at the top, before the pieces concludes:

The biggest question is what choices we and our children should make about energy use. The more dependent we are on carbon-emitting energy sources, the more our climate will change.

If policymakers, businesses and the public are to make smart decisions about climate change, there must be a clear picture of the elements of science that represent robust understanding, elements that remain uncertain and those that depend on future decisions about energy use.

This is the reason for a thorough discussion of scientific findings.

But regardless of how the debate proceeds, it should be clear that opinions or misinformation cannot change the extensive scientific evidence. The atmosphere, the oceans and the land are warming. Humans are contributing significantly to this, and as it continues, it will have a major impact on our society, economy, environment, energy, national security and health throughout, and well beyond, this century.

As climate scientists, we have a responsibility to share our understanding with the public and with policymakers.

But, the future depends not on what scientists have learned and conveyed. Rather, it depends on what society chooses to do with that knowledge.

Dr. James McCarthy is a professor of biological oceanography at Harvard University. Dr. Lisa Graumlich is the dean of the College of the Environment at the University of Washington. Dr. Chris Field is the director of the Department of Global Ecology at the Carnegie Institution for Science. Dr. James Hurrell is a senior scientist with the National Center for Atmospheric Research. All four testified at a recent House Select Committee on Energy Independence and Global Warming hearing, “The Foundation of Climate Science.”

That’s okay for what it is, but let’s remember McCarthy and Hurrell signed the Bali Climate Declaration in late 2007 along with more than 200 of the world’s leading climate scientists, which endorsed the commitment to “limit global warming to no more than 2 ºC above the pre-industrial temperature,” and therefore concluded:

Based on current scientific understanding, this requires that global greenhouse gas emissions need to be reduced by at least 50% below their 1990 levels by the year 2050. In the long run, greenhouse gas concentrations need to be stabilised at a level well below 450 ppm (parts per million; measured in CO2-equivalent concentration). In order to stay below 2 ºC, global emissions must peak and decline in the next 10 to 15 years, so there is no time to lose.

Hear! Hear!