What We Know about Climate

January 25, 2014 7:49 pm4 comments

Dr. Andrew Dessler testifying before U.S. Senate EPW Committee during hearing on President's Climate Action Plan on Jan. 16, 2014.

My name is Andrew Dessler, I am a professor of atmospheric sciences at Texas A&M.  In my testimony, I will review what I think is the most important conclusions that the scientific community has reached in over two centuries of work on climate.

Conclusions

  1. The climate is warming. By this I mean that we are presently in the midst of an overall increase in the temperature of the lower atmosphere and ocean spanning many decades.
  2. Most of the recent warming is extremely likely due to emissions of carbon dioxide and other greenhouse gases by human activities.

This is based on several lines of evidence, including observations of increasing greenhouse gases in our atmosphere and understanding of the greenhouse effect, and a demonstration of the enhanced greenhouse effect can explain the observed warming. For simplicity — in the remainder of my testimony — I am going to refer to this mainstream theory of climate influence as the “standard model.”

The standard model explains just about everything we’ve seen and has successfully predicted phenomena in the climate system.

The standard model, in fact, can explain just about everything we have observed in the climate system, both in the present day and during the geologic record. It has also made many successful predictions, which are the gold standard of science. If you can successfully predict phenomena that are later observed, one can be supremely confident that the theory captures something essential about the real world.

So as an example:

  1. Climate scientists predicted in the 1960s that the stratosphere will cool while the troposphere will warm as result of increased greenhouse gases, and this was observed 20 years later.
  2. In the 1970s, climate models predicted the Arctic will warm faster than the Antarctic. This has also been substantively confirmed.
  3. The water vapor feedback is another fundamental prediction of the standard model that has just recently been observed.

This explains why the bulk of the scientific community is so confident in the standard model. It explains just about everything and it makes many successful predictions.

Now you don’t hear about this very often, because scientists don’t like to talk about things we know. I am uninterested in things we know. I like things we do not know. That’s research. That’s things where we can get stuff done.

I should also add that – obviously — this doesn’t mean that our knowledge is perfect. This is reflected in uncertainty estimates that are provided in the consensus reports.

Now, a caveat: I said above that the standard model explains virtually everything, which means that there is a small number of observations that are not necessarily well explained by the standard model, just as there are a few heavy smokers who do not get lung cancer.

An excellent example of this is the so-called “hiatus,” which has been mentioned several times: the slow warming of the surface temperature record over the last decade or so. This is frequently presented as an existential threat to the standard model. But as I describe below, this greatly exaggerates its implications. Before I explain why, I think it is worth recognizing that skeptics have a track record of overstating the importance of these challenges to the standard model. A few years ago, for example, strong claims were made about the surface temperature record. It was argued that siting issues – for example, a thermometer too close to a building – meant that the surface record was hopelessly biased. This was portrayed as an existential threat to the standard model. Subsequent research, however, has resolved this issue. It is now clear that it was never a threat to the standard model at all.

So why do I think that the hiatus – the slow warming of the last decade – is not much of a threat to the standard model? To begin, a lack of a decadal trend in surface temperatures does NOT mean that the warming has stopped. Observations show that heat continues to accumulate in the bulk of the ocean, indicating continued warming.

Also, in my written testimony, and in the plot that Senator Whitehouse showed, the surface temperature shows frequent periods of short cooling, even while it is undergoing a long-term warming trend.

In addition, one of the Senators said that climate models do not predict periods of no warming. That is not correct. Climate models do predict periods where there is no warming. Now, that does not mean that we understand the hiatus perfectly. I view the hiatus as an opportunity, not as an existential threat. I think short-term climate variability is an area where our understanding could improve, and the hiatus will help us do that. Papers are already coming out on a monthly basis it seems. I suspect that in the next few years our understanding of this phenomena will be greatly improved. At that point, I predict that arguments about the hiatus will disappear, just like arguments of the surface temperature record have.

Now, given the success of the standard model, what does it tell us about the impacts of future climate change?

Before I begin talking about this. I think it is worth discussing the value of what we know, rather than what we don’t know. Focus on what is unknown can lead to an inflated sense of uncertainty. For example, we don’t know the exact mechanism by which smoking cigarettes causes cancer. Nor do we know how many cigarettes you have to smoke to get cancer. Nor can we explain why some heavy smokers don’t get cancer while some non-smokers do. Based on this, you might conclude that we don’t know much about the health impacts of smoking, but that is wrong.

So let me conclude by telling you a few of the certain impacts of climate change:

  1. We know the planet is going to warm. That’s virtually certain.
  2. We know extreme heat events will become more frequent.
  3. We know the distribution of rain fall will change.
  4. We know the seas will rise.
  5. We know the oceans will become more acidic.

We can argue about the things we don’t know, but those are things that are virtually certain. Thank you.

 

Source: Andrew Dessler, Professor of Atmospheric Sciences, Oral Testimony in Review of the President’s Climate Action Plan Hearing Before the U.S. Senate Environment and Public Works Committee, 113th Cong. (Jan. 16 2014). Video of oral testimony available here (testimony begins at 2:58:40). Written testimony available here.

 

 

THE FORUM'S COMMENT THREAD

  • Andy, welcome to the Forum.

    I don’t find anything incorrect in your testimony, although the public has enough confusion with the word “model” that I would have used a different name for the “standard model”.

    You draw a good distinction between the solid science and the things that are still yet to be pinned down. The hiatus, for example, doesn’t magically mean that carbon dioxide has lost its radiative properties and doesn’t seem to have any other such dramatic implications.

    However, would you agree that the behavior of global temperatures over the past 15 years has made it less likely that we’ll hit 6 C above preindustrial by the year 2100, in the absence of substantial mitigation?

    The IPCC basically punts on this one, saying until we know the cause of the hiatus we can’t say anything about its long-term consequences.

    I think that’s a cop-out, for the following reason: One possible implication of the hiatus is that climate sensitivity has been overestimated. Certainly the observation record points toward a lower climate sensitivity now than it did in the year 2000, or even 2007. In the absence of countervailing evidence pointing toward higher sensitivity, shouldn’t this mean that the chances of very high sensitivity have decreased a bit? And shouldn’t that, plus our behind-schedule 21st century warming, make the high end of temperatures in 2100 less likely?

  • Given how fast science on this is moving, I think it’s a little unrealistic to expect the IPCC to have a nuanced and current view of this issue. I’m sure the next IPCC report will have incorporated this issue more completely into our understanding.

    My opinion: I don’t think it’s clear what the “hiatus” tells us about the future trajectory of the climate. If it’s a mode of variability that averages to zero in the long run, then it will alternatively increase and decrease the rate of warming and my have little impact on the temperature around the turn of the next century. Is it possible that it has caused the climate sensitivity to be overestimated? Sure, but I don’t think the peer-reviewed arguments on that are convincing yet.

    • One other thing I would add: the IPCC’s sensitivity estimates have large error bars. I think it’s very very likely that, if it does turn out that models overestimate climate sensitivity, the ultimate ECS will still fall in the range of 1.5-4.5°C.

  • The ability of science to predict what will happen and then observe is a powerful indicator of understanding the system as you indicate. In Glaciology I can see three items that were predicted in the mid-1980’s that we now observe.

    That Pine Island Glacier was particularly vulnerable to warming and it would thin, retreat and accelerate. It has.

    That warming would lead to other Greenland glacier accelerating to speeds like the Jakobshavn. We have seen many Greenland ice sheet outlet glaciers accelerate sginficantly.

    That the rate of disappearing alpine glaciers would increase. It has.

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PUBLIC COMMENT THREAD

  • http://atmo.tamu.edu/profile/JNielsen-Gammon John Nielsen-Gammon

    Andy, welcome to the Forum.

    I don’t find anything incorrect in your testimony, although the public has enough confusion with the word “model” that I would have used a different name for the “standard model”.

    You draw a good distinction between the solid science and the things that are still yet to be pinned down. The hiatus, for example, doesn’t magically mean that carbon dioxide has lost its radiative properties and doesn’t seem to have any other such dramatic implications.

    However, would you agree that the behavior of global temperatures over the past 15 years has made it less likely that we’ll hit 6 C above preindustrial by the year 2100, in the absence of substantial mitigation?

    The IPCC basically punts on this one, saying until we know the cause of the hiatus we can’t say anything about its long-term consequences.

    I think that’s a cop-out, for the following reason: One possible implication of the hiatus is that climate sensitivity has been overestimated. Certainly the observation record points toward a lower climate sensitivity now than it did in the year 2000, or even 2007. In the absence of countervailing evidence pointing toward higher sensitivity, shouldn’t this mean that the chances of very high sensitivity have decreased a bit? And shouldn’t that, plus our behind-schedule 21st century warming, make the high end of temperatures in 2100 less likely?

  • http://atmo.tamu.edu/profile/ADessler Andrew Dessler

    Given how fast science on this is moving, I think it’s a little unrealistic to expect the IPCC to have a nuanced and current view of this issue. I’m sure the next IPCC report will have incorporated this issue more completely into our understanding.

    My opinion: I don’t think it’s clear what the “hiatus” tells us about the future trajectory of the climate. If it’s a mode of variability that averages to zero in the long run, then it will alternatively increase and decrease the rate of warming and my have little impact on the temperature around the turn of the next century. Is it possible that it has caused the climate sensitivity to be overestimated? Sure, but I don’t think the peer-reviewed arguments on that are convincing yet.

    • http://atmo.tamu.edu/profile/ADessler Andrew Dessler

      One other thing I would add: the IPCC’s sensitivity estimates have large error bars. I think it’s very very likely that, if it does turn out that models overestimate climate sensitivity, the ultimate ECS will still fall in the range of 1.5-4.5°C.

  • http://glacierchange.wordpress.com/ Mauri Pelto

    The ability of science to predict what will happen and then observe is a powerful indicator of understanding the system as you indicate. In Glaciology I can see three items that were predicted in the mid-1980’s that we now observe.

    That Pine Island Glacier was particularly vulnerable to warming and it would thin, retreat and accelerate. It has.

    That warming would lead to other Greenland glacier accelerating to speeds like the Jakobshavn. We have seen many Greenland ice sheet outlet glaciers accelerate sginficantly.

    That the rate of disappearing alpine glaciers would increase. It has.

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