How do you prove climate change is a fact? And if it is, what damage can we expect? It’s a thoughtful and fascinating round of climate science questions today.

Got a question about climate science? These next few weeks at Rooted we’re running the Ask a climate scientist series. Keep the questions coming as well, by either leaving one as a comment, or emailing directly to me. Some of you have asked similar questions, so keep an eye out for them, and let me know if that didn’t answer exactly what you were wondering.

These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

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Interesting two-questions-in-one question from Crikey reader Janus Kabulz

  1. Why do so many people (politicians, media etc) say that anthropogenic climate change is a ‘fact’ when (1) the IPCC says that it is “highly likely” that climate change is due to anthropogenic factors, and (2) that it is impossible to prove the null hypothesis, but you can only demonstrate high correlation?
  2. Doesn’t the continued proclamation of “fact” damage scientific reputation and the message that everyone is trying to get out?

Alan Robock, Professor II at Rutgers University, writes:

What is a fact? To a scientist, there are only prevalent theories, and because it is possible that new evidence or understanding, however unlikely, could appear to disprove the current theory, it is hard for us to say something is a fact.

But in my opinion, if something is very likely, it would improve communication with the public to call things facts, and it is good that politicians and media say that. Is it a fact that Earth is round? Is it a fact that gravity pull things downward? Yes. Yes. Is it a fact that Earth is warming? Yes. Is it a fact that the greenhouse gases humans add to the atmosphere cause warming? Yes. Is it a fact that natural causes of climate change (volcanic eruptions and solar variations) cannot explain the warming of the past 50 years? Yes. Is it a fact that the warming caused by the greenhouse gases, particularly carbon dioxide, but also methane, nitrous oxide, and Freon, can explain the warming? Yes. Is it a fact that natural variability of the climate system (chaos) cannot explain the warming? Yes. Is it a fact that there is no explanation other than the effects of humans that can explain the warming? Yes. So I think this is the way, using the word “fact,” that we should explain global warming.

Andreas Schmittner from Oregon State University reviewed this answer and agreed.

Crikey reader Derek Kreckler asks a very pertinent question:

What kind of damage we can expect over the coming 2 – 3 hundred years even if we can reverse the current climate trends.

Dr. Kelly Halimeda Kilbourne,  from the University of Maryland, responds:

There are two ways to interpret this question, so I’ll try to answer both.

If by “reverse the current trends” you mean that we stop emitting greenhouse gases and remove all of the greenhouse gases that we added in the past, then Earth’s temperature would return to normal, and in 200-300 years we would likely have a climate that is very similar to the early 20th Century. Eventually — in a time frame that could be a few years to many thousands of years, we don’t know — the Earth would begin to cool very slowly (over tens of thousands of years) and we would go into the next ice age with global temperatures on the order of a few degrees cooler than today. This scenario is very unlikely however, because we will not be able to stop emitting greenhouse gases immediately, nor currently can we remove all of the greenhouse gases that we have already emitted.

This leads to another interpretation of your question. If you meant that we turn around our current trends in greenhouse gas emissions completely so that we emit no more greenhouse gases in a few decades, then we will have committed ourselves to some warming and some of the changes we already are experiencing, but will avoid the most severe consequences. Over the long term, if we can keep our greenhouse gas emissions to below 450 ppm (we are currently approaching 400ppm from an original pre-industrial value of 280ppm), then the earth will likely come to equilibrium about 1-2 degrees warmer by 200-300 years from now (based on a study by Knutti et al., 2005, Geophysical Research Letters).

The 4th Assessment report of the Intergovernmental Panel on Climate Change has a nice summary figure listing many of the expected impacts at different temperatures (Working Group 2, Summary for Policy Makers, Figure SPM.2). A variety of physical environmental changes will accompany even a modest temperature increase including changes in water availability, sea level, and ocean pH. These physical changes will likely impact people and ecosystems through things like changes in the viability of agriculture in different locations, changes in the distribution of some diseases (e.g. Malaria), changes in species ranges and migration patterns, some species extinctions, increased damage from coastal floods and storms, etc. The changes brought on by greenhouse gases will likely persist for at least 1000 years after we stop our emissions, see the paper by Solomon et al. 2009: Irreversible climate change due to carbon dioxide emissions.

This answer was reviewed by Andreas Schmittner from Oregon State University and Kevin Schaefer from the University of Colorado, who both found it satisfactory.

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Peter Fray
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