CCNF ceased operations in February 2016, and this website is no longer active. Thank you to all the contributors that made the Forum a success. We had a good run! -MAQ

How hot is it going to get and how fast? A chat with Scott Denning on climate sensitivity

July 30, 2014 5:09 pm0 comments

Climate sensitivity has become a regular topic of conversation in the Forum, andas this layman has discoveredit is a central concept to understanding how the climate system works. Most importantly, through climate sensitivity you can get a scientist’s best answer to the million-dollar question of “How hot is it going to get?”

Based on my reading of Forum content, climate sensitivity is fundamentally the ratio between a climate “forcing” (changes in radiation) and a climate “response” (changes in temperature). That is how Dr. Scott Denning described it in his recent post on the topic, titled Cause & Effect. In this first CCNF video interview  the first of many I hope (CCNF is an IRS-confirmed 501(C)(3) nonprofit, and we are fundraising like mad to scale up operations)  Dr. Denning breaks this concept down further.

In the video, he explains how scientists look to various forcings and responses in past changes in climate. These changes include the gradual warming that brought us out of the last ice age, the fairly recent transition from the Medieval Warm Period to the Little Ice Age in Europe, and the recent 1991 Mount Pinatubo eruption that measurably cooled the atmosphere for a brief period. According to Dr. Denning, scientists are independently arriving at basically the the same climate sensitivity of measurement of .8 degrees Celsius per Watt based on all these past changes and through a variety of other methods (e.g., climate models). Moreover, he states that these various studies only have a difference of about .1 degree Celsius per Watt give or take. [Update 7/30/2014: I might have misheard him on that difference. It’s hard to tell hearing it again in the video, but he definitely emphasized the tight shot group.] He adds that this represents a body of scientific work by hundreds of scientists going back to the 19th Century.

In the interview, Dr. Denning also explains that this 0.8 degrees Celsius per Watt translates to an “equilibrium warming” of around 3 degrees Celsius per doubling of CO2 compared to preindustrial levels. I note that this estimate is smack dag in the middle of the IPCC’s climate sensitivity range of 1.5-4.5 degrees Celsius. Later, we get into the question of ‘How long this warming is going to take?’ Other highlights include: his metaphor of CO2 as a gas pedal and temperature rise as the speedometer (emphasizing that there’s a delay but definitely a subsequent response also); the recent discovery of plants eating more and more carbon “for free,” as he puts it; and NASA’s recent launch of the ‘Orbiting Carbon Observatory – 2′ satellite. Check out the interview. If you learn a thing or two, give it a share on social media.

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For those of you who want a quick takeaway, here are the highlights:

Early in the interview, Dr. Denning goes into the First Law of Thermodynamics, which he sums up as “when you add heat to things, they heat up.” He then colorfully observes that calling it the former makes one sound “like an egotistical Victorian brit.” Regarding the question of ‘How long is this going to take?’ (or, more precisely, ‘When will this “equilibrium warming” of 3 degrees Celsius be fully realized?), Dr. Denning had this to say (starting at 16:35 in the video):

Just like anything else, the temperature of the earth approaches that equilibrium temperature closer and closer over time. So you make the big change right away and slowly ramp closer and closer to this equilibrium. Here’s an example, you get on the freeway in your car and punch the gas pedal. Your car doesn’t instantly achieve 100 miles per hour. It goes faster and faster and eventually levels out. So putting CO2 in the atmosphere is like punching the gas pedal of a car, [and] the temperature of the earth is like the speedometer of the car. It takes a while for it to creep up. You may get halfway to equilibrium within 10 years, you may get another half of the way in another 1o years, and another half way in the next ten years and so forth.   But of course, unlike with your car, with CO2, we are continuing to push that accelerator pedal harder and harder and harder. It doesn’t catch up until you stop putting CO2 in the atmosphere and then, after a while, it will catch up. It might be a hundred years. Think of it like that. But not centuries.

I then mention that the sluggishness of the climate response was also noted by Dr. Bart Verheggen in the Forum recently. Dr. Verheggen used the metaphor of a supertanker, writing in The Public Debate on Climate Change that “if we want to change its course, it’s important to start steering the wheel in the desired direction in time.” Here’s the quote from Dr. Verheggen’s piece, The Public Debate, which I paraphrase in the video around 18:18:

[P]reventive emission reduction measures (if desired) would need to be taken before the full extent of the consequences becomes apparent. This means that the longer we wait, the harder it will be to address the consequences of global warming, since by then we have committed ourselves to more warming. As such, those who caused the problem are in the best position to solve it, but since the full consequences will not materialize until much later, they have the least incentive to do so.

I then note in the interview that Dr. Verheggen identified this as one reason why climate change has garnered the descriptor “super wicked problem.” After recognizing that this was going beyond science and into values, I sought a comment from Dr. Denning. Here’s his response:

That is an interesting thought: That we don’t have any incentive to do anything because we are going to be gone. There’s a famous book, titled “Why do Old People Plant Trees.” Why do you save for retirement? Why do you send your kid to the dentist? I mean, you are not going to benefit. Someone down the line is going to benefit, but we do that kind of stuff all the time.

Sadly, I had to cut the values conversation short due to time (the interview is 20 minutes long), but I hope discussion on this topic continues, because unless I am being completely duped here, I believe the Forum is beginning to establish a solid foundation in the science. In my opinion, this makes it as good of a time as any for an initial foray into the values discussion. As noted early on in the Forum by multiple scientists, scientific questions (e.g., ‘How much warming will be caused by X amount of CO2?’) are a different animal than values questions (e.g., ‘Will climate change be bad?'; ‘How bad will it get?'; etc.). The former can be answered by science, but the latter depends on what a person values — whether it be spiritually, morally, ideologically, economically, you name it.

The way I see it, as the co-founding journalist of Climate Change National Forum, both the scientific and values conversations need to be fleshed out before the debate on the policy implications on all this stuff can begin.


NASA graphic of Orbiting Carbon Observatory – 2, successfully launched on July 2, 2014.

Toward the end of the video (around 18:00), I chat with Dr. Denning about the the Orbiting Carbon Observatory-2, a satellite that NASA successfully launched earlier this month. Looking back in hindsight, I should have run this part as a separate video, because the conversation came with some fascinating insights, such as the relatively recent discovery by scientists that plants are actually growing more than they are dying in this high-CO2 world of ours. According to Dr. Denning, these plants have absorbed about half of the carbon emitted by humans since the industrial revolution   “for free”  as he puts it. Here’s the exchange, starting at 19:20 in the video:

MQ: Tell me about this [Orbiting Carbon Observatory – 2] satellite? What is it going to tell scientists? Because I read that it might actually have the ability to monitor carbon down to its actual source on the land?

SD: That’s the point. That’s what the [satellite] is for. So we know how much CO2 is in the atmosphere already. We have 55 years of measurements from the top of Mount Lau… which are excellent in terms of tracking CO2 in the atmosphere year to year, month to month even. We wouldn’t need a satellite to do that. What the satellite is intended to do is monitor tiny little changes in the amount of Co2 from place to place. Let’s say that it makes a daily map of C02. (That’s a little bit of an exaggeration, because it’s really skinny little stripes where there orbit of the satellite is, so it’s not a full map.)

So you can imagine there might be a lot of CO2 downwind from Beijing… . On the other hand, you might imagine there’s not as much CO2 over the Amazon because it is sucking up the CO2 in those trees. So by using these maps, we hope to map out the places where CO2 is being admitted and where CO2 is being soaked up by the oceans and land. […]

Wildfires in Siberia on May 18, 2014. Source: NASA/LANCE/EOSDIS Rapid Response.

Wildfires in Siberia on May 18, 2014. Source: NASA/LANCE/EOSDIS Rapid Response.

But here’s what you may not have heard. Only half of the CO2 emitted by the burning of fossil fuels stays in the atmosphere. So half of the CO2 that has been emitted through the whole history of the industrial revolution–burning coal, oil, and gas–half of that is gone. Where did it go?

MQ: It dissolved into the ocean?

SD: Half of the missing carbon not in the air is in the ocean. You can find it…  But that means the other half of the missing carbon goes into the land. And that was a shock. When I was in graduate school, that was the first time that scientists came to grips with the fact that half of the missing carbon is going into the land. The reason that was a surprise is that it is not enough just to have photosynthesis taking CO2 out of the air, what we are saying is… the growth of new plants is more than the death of old plants. Things are growing faster than they are dying–and this is across the world, which was really a surprise. Almost all ecologists and forestry people before that discovery thought it was the opposite. They thought stuff was dying faster than it was growing. They thought tropical deforestation, plowing the prairie, cutting down the forest to build suburbs — all that stuff was, on the net, turning plant material into CO2. But they were wrong. On the net the CO2 is actually turning into plant material.

And we really would like to know where that is happening, how that is happening, whether it will keep happening, whether that carbon taken up from the atmosphere by the plants will come back anytime soon. So that is what OCO-2 is all about.

MQ: That is possibly a good thing, right? I mean, if there’s an enhanced ability of plants to absorb more [CO2], sounds great.

SD: Yes. Think of it for a second: We are getting a roughly 50% reduction for free by the oceans and land. Wow.

MQ: However, the oceans, that also brings up the issue of ocean acifidication right?

SD: That’s right. That’s right. The CO2 dissolves in the oceans and that changes the ocean, which is a problem. On the other hand, the CO2 in the land and ocean means there’s less CO2 in the air, by half. So that’s a big deal.

MQ: Fascinating discussion. I hope to explore more of these topics and branch out from there. Thank you Dr. Denning for taking the time to talk with us today, and thank you for contributing to Climate Change National Forum. Hope to hear more about OCO-2 in the future.

SD: Sounds great. Nice to talk to you.

About Dr. Denning: Dr. Scott Denning is the Montfort Professor of Atmospheric Science at Colorado State University and also serves as Director of Education and Diversity for CMMAP, the Center for Multiscale Modeling of Atmospheric Processes. He is an author of over 90 publications in the peer-reviewed climate literature, a former editor of the Journal of Climate, and served for five years as founding Science Chair of the North American Carbon Program. He has served on advisory panels for NASA, NOAA, and the National Science Foundation. Dr. Scott Denning joined Climate Change National Forum in February 2014. 

Journalist Michael Quirke interviews atmospheric scientist Dr. Scott Denning on Climate Change National Forum. 7.30.14.

Journalist Michael Quirke interviews atmospheric scientist Dr. Scott Denning on Climate Change National Forum. 7.30.14.


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