Climate change: what do we know?

A short, sharp review of the important stuff everyone should know 

The debate over climate change is so contentious that it’s a little daunting even to write about. The issues are complex. There is vast uncertainty. Disinformation abounds and emotions run high. I couple of years ago, I wrote an article for New Scientist looking at what I thought were some basic issues about long-term energy use. I covered some research (by geophysicists writing in Reviews of Modern Physics) pointing to likely limits in the long run on how much energy we might usefully get from wind and waves. The discussion was essentially about thermodynamics, so I certainly didn’t expect any controversy. I was, of course, immediately attacked in a particularly vicious way by a climate blogger who seemed to think I was a climate change denier. That was a learning experience.

Anyway, on the matter of climate change, this new article in the New York Times is worth a read. Even more so is the thing it refers to, which is a new report on climate change published by the American Association for the Advancement of Science (AAAS). The AAAS is the main professional body for scientists in the US. It’s normally a dry-as-dust organization, sober and measured in all its statements. So what is significant about the new report is its effort to take a strong stand on what we DO know about climate change, as opposed to what we DO NOT know. There’s lots that we don’t know, but that doesn’t mean we know nothing, and this often gets lost in the debate.

A couple of years ago, climate scientist Michael Mann took Nate Silver to task for overlooking just this point. Silver had interviewed Mann in preparation for writing his book The Signal and the Noise, but, according to Mann, failed in the book to distinguish clearly what IS known about climate science from what IS NOT known:

I had emphasized the importance of distinguishing the true uncertainties in climate science (and there are plenty e.g. the influence of warming on hurricanes, how the El Nino phenomenon might be affected, or how regional patterns of rainfall may change) from the manufactured uncertainties and myths typically promoted by climate change deniers and contrarians (e.g. "how come there has been no warming since 1998?" -- the answer is that, of course, there has been). I stressed how important it is, when scientists communicate to the public, to make clear that while there are many details that are still uncertain, the big picture (that humans are warming the planet and changing the climate, and that far larger and potentially more dangerous changes loom in our future if we don't act) is not.

…Nate's chapter on climate change (Chapter 12: "A Climate of Healthy Skepticism") is marred by straw man claims that don't stand up to scrutiny. These include the assertion that (a) climate scientist James Hansen's famous 1988 predictions overestimated global warming (they didn't), that (b) "the Intergovernmental Panel on Climate Change (IPCC) settles on just one forecast that is endorsed by the entire group" (pure nonsense -- even the most casual reading of the IPCC reports reveals that great care taken to emphasize the non-trivial spread among model predictions, and to denote regions where there is substantial disagreement between the projections from different models) and that (c) "relatively little is understood" about the El Nino cycle (here I imagine that Nate might have misinterpreted our own discussion about the matter; I explained in our discussion that there are still open questions about how climate change will influence the El Nino phenomenon -- but that hardly means that we know "relatively little" about the phenomenon itself! In fact, we know quite a bit about it).

So, what does the new AAAS report assert about what we DO know? Several key things:

First, that climate change is real and happening right now, that human practices are a significant cause of it, and that virtually all scientists agree on this. As the report says:

Based on well-established evidence, about 97% of climate scientists have concluded that human-caused climate change is happening. This agreement is documented not just by a single study, but by a converging stream of evidence over the past two decades from surveys of scientists, content analyses of peer-reviewed studies, and public statements issued by virtually every membership organization of experts in this field. Average global temperature has increased by about 1.4˚ F over the last 100 years. Sea level is rising, and some types of extreme events – such as heat waves and heavy precipitation events – are happening more frequently.

Second, that our actions are pushing the Earth-system towards potentially irreversible changes, many of which may occur quite abruptly, as tipping point events:

Earth’s climate is on a path to warm beyond the range of what has been experienced over the past millions of years.[ii] The range of uncertainty for the warming along the current emissions path is wide enough to encompass massively disruptive consequences to societies and ecosystems: as global temperatures rise, there is a real risk, however small, that one or more critical parts of the Earth’s climate system will experience abrupt, unpredictable and potentially irreversible changes. Disturbingly, scientists do not know how much warming is required to trigger such changes to the climate system.

Third, and finally, that it will be cheaper and more effective to take action sooner rather than later. Waiting, that is, is not likely to be a good strategy, and will only increase risks:

Waiting to take action will inevitably increase costs, escalate risk, and foreclose options to address the risk. The CO2 we produce accumulates in Earth’s atmosphere for decades, centuries, and longer. It is not like pollution from smog or wastes in our lakes and rivers, where levels respond quickly to the effects of targeted policies. The effects of CO2 emissions cannot be reversed from one generation to the next until there is a large- scale, cost-effective way to scrub carbon dioxide from the atmosphere.

These are fairly strong statements coming from a group as cautious as the AAAS, and I think they reflect the frustration of scientists that what they do know about climate change, the knowledge that reflects the vast consensus that exists among scientists in this area, often gets swamped out in discussions in the media. It is simply too easy for special interests to throw grenades of doubt into the arena by exploiting acknowledged uncertainties and amplifying them to make it seem that everything is uncertain, which it isn’t.

The summary of the document is careful to say that scientists don’t claim to know what precisely we should do, or to have a right to any special place in the policy making process. But science is about knowledge, and scientists do want to get what they do know heard by the population at large:

As scientists, it is not our role to tell people what they should do or must believe about the rising threat of climate change. But we consider it to be our responsibility as professionals to ensure, to the best of our ability, that people understand what we know…

The full report can be read here.