How Will Climate Change Impact the U.S.? MIT Alumni Explain.

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Two MIT alumni provided a behind-the-scenes look at the National Climate Assessment (NCA), a document released May 6 by the While House. The report summarizes present-day and future impacts of climate change on the United States and predicts that U.S. average temperature could rise more than 4°F (2.2°C) over the next few decades and that flooding, wildfires, and droughts will increase.

Convening lead author John Walsh SM ’74, PhD ’79, professor and chief scientist at the University of Alaska Fairbanks, and contributing author Thomas Knutson SM ’89, research meteorologist at the NOAA Geophysical Fluid Dynamics Laboratory spoke to Slice of MIT about their role in the report. Read the full interview or click on a question to jump to the response.

1. Is there any reason to be optimistic about the future of Earth’s climate?
2. What do you hope this report accomplishes?
3. What surprises—if any—did you find in your research?
4. What are the biggest variables for climate change in the future?
5. What role did the White House play in putting the report together?
6. Aside from the predicted rising temperatures and rising sea level, are there other areas that should merit more attention?
7. What has been the response from skeptics of human-made climate change?
8. What expectations do you have for this research going forward?

Walsh: “In the long run, yes. We’re building an awareness of the seriousness of what’s happening right now. But the slope of awareness isn’t as large as we need. It doesn’t seem like we’ll get any action or policy changes over the next few years. It’s going to take more hits on the climate system before real action is taken. So I’m optimistic in the long term, pessimistic in the short term.” ^{BACK TO TOP}

What do you hope this report accomplishes?

Walsh: “Wake up the general public to the reality of climate change. If the public is behind some steps towards mitigation, I think that’s a good thing.

The report is not meant to make policy recommendations, but to inform policy and decision makers by presenting the best evidence and the best science that we can. We pointed out that there are risks associated with the current trajectory of Earth’s climate system.”

Knutson: “I hope readers will gain an appreciation for the various changes happening in the climate system and what types of changes scientists are pretty confident are resulting from human influence, versus other changes where scientists are not so confident.

It’s important not to lump all types of observed climate changes together. This information is meant to give the public, policymakers, and other scientists some guidance for what to expect as humans continue to increase the greenhouse forcing and further modify the earth’s climate. It’s important to give readers the best scientific evidence, no matter what direction it points.” ^{BACK TO TOP}

What surprises—if any—did you find in your research?

Walsh: “I was surprised by the decisiveness of the information—the high precipitation, the flood events, and the heat waves. Because of the major floods in the last year, that message came through quite effectively. The fact that we’ve had severe floods and major heat waves, in a way, really adds some punch to the whole assessment.”

Knutson: “Hurricanes and how they relate to climate change is a very tricky issue. It can be very different from temperature, where climate scientists are relatively confident that human-caused emissions of greenhouse gases have led to most of the warming over the past half century. We don’t have the same situation for hurricane activity.

If you study U.S. landfalls of hurricanes from as far back as the late 1800s until now, there’s really no notable trend to report on. There is an increasing trend in hurricane activity since the 1970s, but that’s too short a period to tell us much about whether there is a century-scale trend or not.

Climate models that we use are suggesting that hurricanes will become slightly stronger—perhaps five percent more intense—by the end of the 21st century. For the Atlantic region, there may be fewer storms overall, but models suggest there will be more of the most intense Category 4 or 5 storms.” ^{BACK TO TOP}

What are the biggest variables for climate change in the future?

Walsh: “The rate of sea level rise. We’re confident that sea level will rise. But whether it will rise by one foot or four feet in the next 100 years is unclear.

The biggest uncertainty is what happens to the ice sheets in Greenland and Antarctica and how much their loss of ice will contribute to sea level rise. We don’t understand the dynamics of these ice sheets and we don’t know how much they’ll contribute to sea level rise over the next few years.” ^{BACK TO TOP}

What role did the White House play in putting the report together?

Walsh: “Not a direct role. The White House drove the mandate to put this all together, which is now a living document that will live online. Everyone kept the politics at arm’s length. No one told us what to say or how to say it. They left it up to the scientists to put the package together.

The Executive Branch worked with the coordinating body, the U.S. Global Change Research Program. Federal agencies did review the report and their comments were addressed by the authors.” ^{BACK TO TOP}

Aside from the predicted rising temperatures and rising sea level, are there other areas that should merit more attention?

Walsh: “Yes, the pause in global warming. Overall temperatures have not increased in the last decade, but historical records have shown occasional decades of absence of warming. These things are a fact of life in the climate system.

In the climate science chapter, we gave a lot of attention to that particular issue. But it’s important to note that there was an unusually large uptake of heat in the ocean over the past decade or so.” ^{BACK TO TOP}

What has been the response from skeptics of human-made climate change?

Walsh: “I’ve interacted with them along the way. Evidence about the increase in heavy precipitation events and the increase and frequency of heat waves is something that many of them are now acknowledging in the realities of the data.” ^{BACK TO TOP}

What expectations do you have for this research going forward?

Knutson: “The past contains important clues for future. If we’re able to detect a strong trend in past tropical cyclone data, the fact that we can already detect such a trend gives us more confidence with a future protection of a similar trend. For instance, we know that we have had a rise in global mean temperatures since the 1870s. The reason we have so much confidence that global mean temperatures will continue to rise over the coming decades is because of our detection of long-term past rising changes.

There is not real strong evidence for past century-scale increases in hurricane activity though. One of the few regions where we see a trend is in Northeast Australia, where the trend has been downward. But our data records are relatively limited, so there is still a lot of uncertainty on how tropical cyclones changed in various regions over the past century as the climate warmed. This is one reason why we are somewhat cautious in our statements about future expected changes in tropical cyclone activity.” ^{BACK TO TOP}