If scientists can only claim to be 90% sure of man-made climate change, does this make it a judgment call rather than fact? Does the same concept apply to other scientific issues and theories? Plus, how it is even proved what is man-made and what isn’t when it comes to climate change? It’s an interesting grab-bag of climate questions being answered by climate scientists today.
Got a question about climate science? Crikey’s environment blog Rooted has been running the Ask a climate scientist series for several months though, and it’s well worth trawling through the archives if you haven’t already.
The answers to these questions come 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.
This service is currently on a short hiatus — it was only a pilot trial to begin with — and the AGU is evaluating the success of the service so far. Which is why there has been a long delay between ‘Ask a climate scientist’ posts, I didn’t want to post questions without knowing what was happening. But all signs point to the service being restarted again soon, so figured we’d reopen the debate.
If you’ve got a question, feel free to email it through to me and once the service is restarted, we can email them all through.
Crikey reader Martin asks:
We’re told the IPCC is “90% certain” that the majority of the climate change we’re experiencing is man-made. How is this figure determined? How can you be “90%” certain about something — as opposed to 89% or 68.3%? Isn’t this just a judgement call? And why don’t we see the same thing said about other sciences, e.g. that because of say, the Pioneer anamoly, we’re only “90% certain” about our theory of gravity?
Jim Bouldin, research ecologist at the University of California responds:
First, you are assumedly referring to the AR4 statements that refer to the last 50 years only. These judgements are the expert opinion of the AR4 authors, based on their knowledge and integration of a wide variety of observational and model data bearing on the topic, which are detailed in the Report. These include a systematic evaluation, using models and observational data, of the likelihood that various different, potential climate drivers would have caused the changes observed. Because these data derive from widely varying sources, time periods, and analytical methods, they are not readily integrated into a standard statistical probability framework, although this is certainly a goal for the science. This approach is the essence of expert judgment in any field, and has a long-standing history in most complex sciences (climate science, geology, ecology, etc.).
The IPCC issued specific guidelines to AR4 lead authors regarding how to deal with uncertainty quantification. They have recently released similar guidelines for the upcoming AR5. Those guidelines should be consulted, as they go in to much more detail than can be summarized here. They can be read here and here.
Because the stated likelihoods in the Report are not from well-defined statistical distributions, the IPCC has defined the break points fairly widely. So, “very likely” corresponds to >= 90% confidence, while “likely” to >= 66% confidence. There is no attempt to discriminate at the fine levels you mention, nor to give the impression of doing so.
Lastly, we do in fact see probabilistic assessments of likelihood in all types of science, all the time. It may not be evident to the average person, but some type(s) of statistical analysis of probabilities and likelihoods underlies the vast majority of science. We are constantly weighing evidence against concepts, by various methods. So, the premise of the last question is faulty.
Reviewed by Robert Jacob, computational climate scientist, Argonne National Laboratory.
Crikey reader Joe Purcell asked:
I would like to learn more about ‘forcings’ . People often say “but the climate has changed before, why do scientists conclude that on this occasion it is caused by human activity?”
Gavin Schmidt from the NASA/Goddard Institute for Space Studies responds:
‘Forcings’ are just another term for drivers of climate change and can arise from all sorts of different sources. One ‘forcing’ would be the changes in the sun’s brightness for instance as a function of the 11 yr solar cycle. Another natural forcing arises from the impact of large volcanic eruptions, which can put significant amounts of reflective ‘aerosols’ (or particulates) in the stratosphere. Going further back into the past, the closing of isthmus of Panama about four million years ago (because of plate tectonics) was a forcing that altered climate and ocean circulation in the North Atlantic.
Over the twentieth century though, it is human activity that has provided most of the forcings. That activity has changed the amount of CO2 and other greenhouse gases in the air, altered the concentration of aerosols, and chopped down forests that change how sunlight is reflected from the surface. Each of these factors has been a forcing, and we are pretty sure that the net effect of all of these changes has been to drive the climate towards warming. The natural forcings (from the sun and volcanic activity) are mostly neutral over this time period, perhaps even pushing towards cooling in the most recent decades.
The recent changes in CO2 or methane have been caused by human activity — mainly the burning of fossil fuels, deforestation for CO2 and agriculture, landfills and mining for CH4 — and so while these constituents have varied in the past (over the ice age cycles for instance), the reasons they varied then are not why they are changing today.
This answer was reviewed by Robert Jacob from the Argonne National Laboratory