Climate change is already here. How bad it gets is still up to us

A lead author of the new IPCC report explains what we know and how we know it

Published March 31, 2014 7:15PM (EDT)

If you want to understand where mainstream science stands on climate change, the U.N.'s Intergovernmental Panel on Climate Change is the best place to start. Its latest report reflects a thorough analysis of the peer-reviewed science and a harsh, but admittedly conservative, overview of predictions for the future. It includes input from a total of 309 lead authors, representing 70 nations -- and helped along by 436 contributing authors and 1,729 expert and government reviewers.

Salon spoke with Dr. Noah Diffenbaugh, an associate professor and senior fellow at the Stanford Woods Institute for the Environment and one of the lead authors of the new report, which was released Monday morning. The challenge in putting it together, Diffenbaugh told Salon, wasn't in trying to determine whether climate change is happening: Its impacts, as climate scientists have made clear, are already being felt. On the contrary, he said, there's so much known about climate change that the real task was distilling the scientific knowledge into a clear and concise message. That message: Climate change is here, and if we don't take drastic action to limit warming, its effects are going to get worse.

First, tell us a little bit about your contribution to the report. What was the process like for you?

I’ve been a lead author on the North American chapter [more on that here], and I have also coordinated a cross-chapter effort to integrate the physical climate information across a number of regional chapters. And I have found this to be a very inspiring and fulfilling experience, particularly to observe  how committed so many authors are to conducting such a disciplined and accurate  assessment of the scientific literature while at the same time presenting it in a way that is accessible for the decision-makers that have commissioned us with conducting the report.

What were some of the biggest challenges in putting something like that together? 

Certainly, the biggest challenge is that there’s so much known about climate change. Our job is to assess the literature that has appeared in the last seven years since the last IPCC report, and we’ve really seen a huge amount of growth in the state of knowledge about the impacts of climate change. So we have a huge literature to assess and that’s been one of the most fulfilling aspects, but also one of the most challenging.

Can you give a quick explanation of how climate modeling works? What is it able to tell us about how climate change is affecting us now, and about what the future effects might be? 

So when I was in eighth grade, in science class, I conducted experiments on corn plants. If you water some and don’t water others, and you put some in the light and put some in the closet, in the dark, you can see what happens to the corn plants as a result of these experiments. And we would really like to be able to run experiments like that on the global climate system. But the problem is that we can’t, we can’t run those experiments. And so we use climate models to run those experiments that we’d like to run in the real world. These models are based on the laws of physics -- they literally start with Newton’s Laws -- and they allow us to simulate the climate system and run the experiments that we would like to be able to run in the real world.

So how would you characterize the level of confidence with which they're able to tell us about climate change?

We use climate models in the same way that other scientists conduct their science, in that we create hypotheses and we try to use real observations to test those hypotheses. So we run the climate model experiments for the past using the known concentrations of greenhouse gases, and that creates a hypothesis about how the climate system responds to the human input of greenhouse gases into the atmosphere. And then we’re able to compare those results of the climate model experiments with the observations from the real world.

As far as North America -- your focus -- goes, what are some the specific things to look out for that are highlighted in the report?

We see in North America that climate change is already happening and we’re already experiencing impacts. We’re already seeing increases in some extreme events such as severe heat and heavy downpours and extreme flooding during storm surges. We’re also seeing decreases in snowpack and earlier timing of snowmelt in many parts of the Western United States. So these climate changes are already happening and we’re already feeling the effects.

What’s the future outlook for the catastrophic impact — when things are going to reach that point of no return?  Do you have a good understanding of how close we are?

We have a lot of evidence that if global warming continues along its current trajectory, that we’re likely to see high-impact climate change in the United States. We’re likely to see really large increases in severe heat, to the point that almost every summer in likely to be hotter than what used to be the historically hottest summer. We’re likely to see substantial increases in sea level that will increase the risk of extreme storm surge during storm events. And in the United States, we have large populations that live near the coast. In terms of agriculture we know that many crops are sensitive to severe heat and that continued global warming is likely to increase the stress from severe heat on a number of important crops in the United States.

It’s important to emphasize that we have a lot of opportunities to manage these risks in the climate system. We have opportunities to manage how much global warming occurs and we have opportunities to manage our exposure and vulnerability to build resilience to climate stresses.

What is the most significant action we could be taking right now to address these issues?

As a scientist, it's not my role to make any policy prescriptions for society, but it is clear, in terms of just the physics of how the world works, that we will experience much greater increases in extreme events from high levels of global warming than we will within the target for global warming that has been set by the United Nations, for example [2 degrees Celsius]. So the difference between 2 degrees Celsius for global warning and 4 degrees Celsius of global warming is really substantial in terms of the risk of extreme events: the risk of severe heat, the risk of heavy precipitation, the risk of low snow years and the risk of extreme floods. These will all be much reduced in a world with 2 degrees Celsius of global warning compared to 4 degrees.

Without giving any specific recommendations, then, what are you hoping the reaction to the report will be like? What do you want people to take away from it? 

I think the most important message that the public can take away from this report is that a large number of scientists have taken a look at a large body of evidence and we know that climate change is occurring. We know we’re already experiencing the impacts, and we know that there are opportunities to manage those risks in terms of the level of global warming, and in terms of our vulnerability to climate change.

By Lindsay Abrams

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Climate Change Extreme Weather Greenhouse Gases Ipcc United Nations