(Part of the How to Talk to a Global Warming Skeptic guide)

Objection: Climate is complicated and there are lots of competing theories and unsolved mysteries. Until this is all worked out, one can’t claim there is consensus on global warming theory. Until there is, we should not take any action.

This is similar to the “global warming is a hoax” article, but at least here we can narrow down just what the consensus is about.

Answer: Sure there are plenty of unsolved problems and active debates in climate science. But if you look at the research papers coming out these days, the debates are about things like why model predictions of outgoing longwave radiation at the top of the atmosphere in tropical latitudes differ from satellite readings, or how the size of ice crystals in cirrus clouds affect the amount of incoming shortwave reflected back into space, or precisely how much stratospheric cooling can be attributed to ozone depletion rather than an enhanced greenhouse effect.

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No one in the climate science community is debating whether or not changes in atmospheric CO2 concentrations alter the greenhouse effect, or if the current warming trend is outside of the range of natural variability, or if sea levels have risen over the last century.

This is where there is a consensus.

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Specifically, the “consensus” about anthropogenic climate change entails the following:

  • the climate is undergoing a pronounced warming trend beyond the range of natural variability;
  • the major cause of most of the observed warming is rising levels of the greenhouse gas CO2;
  • the rise in CO2 is the result of burning fossil fuels;
  • if CO2 continues to rise over the next century, the warming will continue; and
  • a climate change of the projected magnitude over this time frame represents potential danger to human welfare and the environment.

While theories and viewpoints in conflict with the above do exist, their proponents constitute a very small minority. If we require unanimity before being confident, well, we can’t be sure the earth isn’t hollow either.

This consensus is represented in the IPCC Third Assessment Report, Working Group 1 (TAR WG1), the most comprehensive compilation and summary of current climate research ever attempted, and arguably the most thoroughly peer reviewed scientific document in history. While this review was sponsored by the UN, the research it compiled and reviewed was not, and the scientists involved were independent and came from all over the world.

The conclusions reached in this document have been explicitly endorsed by …

  • Academia Brasiliera de Ciências (Bazil)
  • Royal Society of Canada
  • Chinese Academy of Sciences
  • Academié des Sciences (France)
  • Deutsche Akademie der Naturforscher Leopoldina (Germany)
  • Indian National Science Academy
  • Accademia dei Lincei (Italy)
  • Science Council of Japan
  • Russian Academy of Sciences
  • Royal Society (United Kingdom)
  • National Academy of Sciences (United States of America)
  • Australian Academy of Sciences
  • Royal Flemish Academy of Belgium for Sciences and the Arts
  • Caribbean Academy of Sciences
  • Indonesian Academy of Sciences
  • Royal Irish Academy
  • Academy of Sciences Malaysia
  • Academy Council of the Royal Society of New Zealand
  • Royal Swedish Academy of Sciences

… in either one or both of these documents: PDF, PDF.

In addition to these national academies, the following institutions specializing in climate, atmosphere, ocean, and/or earth sciences have endorsed or published the same conclusions as presented in the TAR report:

If this is not scientific consensus, what in the world would a consensus look like?

(Addendum: One could legitimately argue that such policy statements by necessity hide possibly legitimate internal debate while trying to present unity of position. Science is ultimately determined in peer reviewed journals. Fortunately, there is a bit of research that looked specifically at this very question — the subject of another guide entry.)