Friday’s release of the latest international assessment of climate change has more firmly than ever placed its cause at the feet of humans. The Intergovernmental Panel on Climate Change Fifth Assessment stated: “Human influence on the climate system is clear.
“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the report said.
Gary Yohe, the Huffington Foundation professor of economics and environmental studies at Wesleyan University, has been involved with the IPCC since the early 1990s, though not with the group that produced this document. Since 2011 he has also served as vice chair of the National Climate Assessment Development and Advisory Committee.
“The Intergovernmental Panel on Climate Change, throughout its history,” has actually been very conservative in the conclusions that it draws,” Yohe said noting the significance of the report’s finding that the evidence of human influence has grown since the last report a half-dozen years ago.
The certainty climate change is caused by humans is now between 95 and 100 percent, the report said, stating: “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
In warning of increasing levels of greenhouse gases, the report stated: “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”
It also recommended a limit on how much carbon dioxide would be too much.
This was only the summary (the full report will be released next week) from one of four working groups within the IPCC – working group 1, which evaluates scientific evidence. Working group 2, of which Yohe is a member, will look at impacts of climate change; its fifth assessment is due in March.
What follows is an edited transcript of a conversation with Yohe after the Friday report was released.
What would you consider the most important points to come out of this summary from the first working group?
The first one is that the scientific confidence on conclusions that have been building over the last decade or two is going up and up. And so this is a strengthening, like Sec. [John] Kerry says, this is a strengthening of the overall scientific conclusions that were reported in the fourth assessment report, in the third assessment report and so on and so forth. So some will look at this as the same old news to which I would respond — that’s pretty much the point. We’ve been saying this for a decade or two. Our confidence is the scientific evidence is getting stronger. And a lot of people have spent an awful lot of time doing work on this and they haven’t come up with any strong reason to believe that these conclusions are wrong. And in fact they’ve come up with increasing evidence that these conclusions are exactly right.
Why is it important to know for sure that it’s attributed to human influences?
One is if you think about how to respond and you say, “Well a really good idea would be to reduce the emissions of heat-trapping gases,” you have to be able to attribute the observed warming and the commitment to even higher warming to human influences to believe that reducing the emission of greenhouse gases will make any difference at all. That’s the message from working group 1 to frame what will come out of working group 3 on mitigation sometime late next spring or early summer.
The other is, if you’re thinking about adaptation and responding in the way you design buildings, the way you design development codes and things like that, you can respond and adjust to what you observe locally if you have relatively short time periods and you can make adjustments as you observe more things. But if you’re thinking about putting a boardwalk back in New Jersey and that’s a long-term investment in tourism in New Jersey, you really do have to take into account a very long-term future and the possibilities of climate change.
The only way you can do that is to try to think about what is driving the climate change and what would make those circumstances worse or not so bad, and link that back to the emission of heat-trapping gases from around the world. That gives you something real that you can keep track of, but it also gives you a frame within which you can make some risk judgments about whether or not you want to rebuild the boardwalk exactly where it was; whether you want to let people rebuild their houses exactly where they were. Stuff like that.
This report sets up sort of the scientific backbone, your group works more on the impacts.
Yeah. And what it means to human systems and natural systems and things that humans have decided that they worry about.
Working group 1 with respect to what does it mean to families and individuals, people in Connecticut or anywhere around the world, is only half finished. It will be the working group 2 report that speaks directly to the consequences of the specifics and the generalities that come out of the working group 1 report. It is premature to take working group 1 and say, “OK, we’ve got all we need.” You only have half of what you need.
Thinking ahead to your group’s report, which is coming in March, what is the overall message for local governments, policymakers, national governments, regional governments in terms of how they view all this?
None of the working groups; nothing in IPCC, nothing in the National Climate Assessment can tell these people what do. We cannot be policy prescriptive. But we can look at what working group 1 said and identify increased levels of likelihood consequence that this is going to happen or that is going to happen, to justify in the discussions of what comes out of working group 2; why anybody should worry about this. It is a calculation of risk. You can, just in your brain say, likelihood has gone up thanks to working group 1; working group 2 is going to survey consequence, take into account all sorts of compounding factors in the way humans behave, how they respond, how they adapt naturally and frame the consequence part of the risk calculation.
Knowing what you know from working group 1 and what you have been looking at in working group 2, if you were to make a list of recommendations to officials in Connecticut, officials in New England and the Northeast about things they need to consider moving forward as far a public policy goes to deal with climate change, what would be some of the priorities you’d want to give them?
I would want them to focus a lot on the manifestation of climate in the implications with respect to the intensity and frequency of extreme weather. So for farmers, they need to worry about drought, but they also need to worry about five inches of rain in four hours.
For people who live along the coastline, they need to worry about the increased intensity of coastal storms due to sea level rise even if the intensity and frequency of those storms doesn’t change – just the way they show up on the coastline — is something they really, really need to take into account. 
Then you add to that some inference about changes in the likelihood of intensity and frequency, for which confidence is a lot lower than 95 percent that humans are to blame for the warming, but very far from zero and probably close to – well – more-likely-than-not. And so those are things that adopting a risk management perspective on how you look at all of the planning decisions that you undertake need to be accounted for.
These people do risk management all this time. This is just a new source of risk that they have to take into account and the time frame over which they are making the plans is really quite critical.
You are an economist; you look at risk and numbers all the time. How risky a situation do we have? How scared should we be?
I think we should be very scared. I think that livelihoods will be in jeopardy; coastal property will be in jeopardy with likelihoods that are higher than anybody thought – certainly NOT calibrated from the past 30 years of weather data on the old climate. There’s a new climate coming. So I think people need to worry a good deal about that.
Imbedded in this report though is — in a sense — some good news, which is if we broke the climate, we can possibly fix it.
That is good news.
The message is not – we are going to hell in a handbasket and there’s nothing we can do about it. It is that there’s a wide range of futures out there. And we can work to make the ones that we like much more likely and the ones that look really, really scary much less likely by undertaking adaptation and mitigation.
The response to throw up your hands, there’s nothing we can do would logically be, from an economist perspective: OK, let’s just tramp on the accelerator, enjoy the ride, and when we get to the cliff and fall off then that was the way it was, but we were going to do that anyway. That is not the message that should be coming out of this.
The message that should be coming out of this is that if we drive a little bit more carefully, slow down, take note of weather conditions and road conditions and things like that, maybe we can not fall off of that cliff.