Global warming could be null and void.

• Position: Climate Change

The debate largely may be drawn along political lines, but the human role in climate change remains one of the most controversial questions in 21st century science.

Writing in WIREs Climate Change, Kevin Trenberth, from the National Center for Atmospheric Research, Boulder, Colo., argues that the evidence for anthropogenic climate change is so clear that the burden of proof should lie with research that seeks to disprove the human role.

In response to Trenberth's contention, a second review, by Judith Curry, Georgia Institute of Technology, Atlanta, focuses on the concept of a "null hypothesis," the default position taken when research is carded out. Currently, the null hypothesis for climate change attribution research is that humans have no influence.

"Humans are changing our climate. There is no doubt whatsoever," insists Trenberth. "Questions remain as to the extent of our collective contribution, but it is clear that the effects are not small and have emerged from the noise of natural variability. So, why does the science community continue to do attribution studies and assume that humans have no influence as a null hypothesis?

"Scientists must challenge misconceptions in the difference between weather and climate while attribution studies must include a human component," concludes Trenberth. "The question...