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This item was posted on April 10, 2009, and it was categorized as Antarctica, Climate, Climate Change, Climate change policy, Global Warming, Tipping points, climate variability, climatology, greenhouse gases, sea level rise.
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NOAA scientists agree the risks are high, but say Hansen overstates what science can really say for sure

NASA climatologist at the University of Colorado's World Affairs Conference (Photo: Tom Yulsman)

Jim Hansen at the University of Colorado’s World Affairs Conference (Photo: Tom Yulsman)


Speaking to a packed auditorium at the University of Colorado’s World Affairs Conference on Thursday, NASA climatologist James Hansen found a friendly audience for his argument that we face a planetary emergency thanks to global warming. 

Despite the fact that the temperature rise has so far been relatively modest, “we do have a crisis,” he said.

With his characteristic under-stated manner, Hansen made a compelling case. But after speaking with two NOAA scientists today, I think Hansen put himself in a familiar position: out on a scientific limb. And after sifting through my many pages of notes from two days of immersion in climate issues, I’m as convinced as ever that journalists must be exceedingly careful not to overstate what we know for sure and what is still up for scientific debate. 

Crawling out on the limb, Hansen argued that global warming has already caused the levels of water in Lake Powell and Lake Mead — the two giant reservoirs on the Colorado River than insure water supplies for tens of millions of Westerners — to fall to 50 percent of capacity. The reservoirs “probably will not be full again unless we decrease CO2 in the atmosphere,” he asserted.

Hansen is arguing that simply reducing our emissions and stabilizing CO2 at about 450 parts per million, as many scientists argue is necessary, is not nearly good enough. We must reduce the concentration from today’s 387 ppm to below 35o ppm.

“We have already passed into the dangerous zone,” Hansen said.  If we don’t reduce CO2 in the atmosphere, “we would be sending the planet toward an ice free state. We would have a chaotic journey to get there, but we would be creating a very different planet, and chaos for our children.” Hansen’s argument (see a paper on the subject here) is based on paleoclimate data which show that the last time atmospheric CO2 concentrations were this high, the Earth was ice free, and sea level was far higher than it is today.

“I agree with the sense of urgency,” said Peter Tans, a carbon cycle expert at the National Oceanic and Atmospheric Administration here in Boulder, in a meeting with our Ted Scripps Fellows in Environmental Journalism. “But I don’t agree with a lot of the specifics. I don’t agree with Jim Hansen’s naming of 350 ppm as a tipping point. Actually we may have already gone too far, except we just don’t know.”

A key factor, Tans said, is timing. “If it takes a million years for the ice caps to disappear, no problem. The issue is how fast? Nobody can give that answer.” 

Martin Hoerling, a NOAA meteorologist who is working on ways to better determine the links between climate change and regional impacts, such as drought in the West, pointed out that the paleoclimate data Hansen bases his assertions on are coarse. They do not record year-to-year events, just big changes that took place over very long time periods. So that data give no indication just how long it takes to de-glaciate Antarctica and Greenland. 

Hoerling also took issue with Hansen’s assertions about lakes Powell and Mead. While it is true that “the West has had the most radical change in temperature in the U.S.,” there is no evidence yet that this is a cause of increasing drought, he said.

Flows in the Colorado River have been averaging about 12 million acre feet each year, yet we are consuming 14 million acre feet. “Where are we getting the extra from? Well, we’re tapping into our 401K plan,” he said. That would be the two giant reservoirs, and that’s why their water levels have been declining.

“Why is there less flow in the river?” Hoerling said. “Low precipitation — not every year, but in many recent years, the snow pack has been lower.” And here’s his almost counter-intuitive point: science shows that the reduced precipitation “is due to natural climate variability . . . We see little indication that the warming trend is affecting the precipitation.”

In my conversation with Tans and Hoerling today, I saw a tension between what they believe and what they think they can demonstrate scientifically.

“I like to frame the issue differently,” Tans said. “Sure, we canot predict what the climate is going to look like in a couple of dcades. There are feedbacks in the system we don’t understand. In fact, we don’t even know all the feedbacks . . . To pick all this apart is extremely difficult — until things really happen. So I’m pessimistic.”

There is, Tans said, “a finite risk of catastrophic climate change. Maybe it is 1 in 6, or maybe 1 in 20 or 1 in 3. Yet if we had a risk like that of being hit by an asteroid, we’d know what to do. But the problem here is that we are the asteroid.”

Tans argues that whether or not we can pin down the degree of risk we are now facing, one thing is obvious: “We have a society based on ever increasing consumption and economic expectations. Three percent growth forever is considered ideal. But of course it’s a disaster.” 

Hoerling says we are living like the Easter Islanders, who were faced with collapse from over consumption of resources but didn’t see it coming. Like them, he says, we are living in denial.

“I think we are in that type of risk,” Tans said. “But is that moving people? It moves me. But I was already convinced in 1972.”




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This thing has 10 Comments

  1. Buckwheat
    Posted April 11, 2009 at 8:09 am | Permalink

    Hansen, you are full of beans. Where did the last glacier go? The Wisconsionian glaciar and the subsequent aurignacian oscillations. Is that data entered into in your pseudo professional projections. Crap science is pseudoscience and you have no data foundation for yor projections if you do not include at least the last three glaciations.

  2. novoburgo
    Posted April 11, 2009 at 8:59 am | Permalink

    This entire article is leaves me shaking my head. According to Tans he agrees with “the sense of urgency” but not “a lot of the specifics.” Also, he admits to a problem with timing, stating that “If it takes a million years for the ice caps to disappear, no problem.” This must be the epitome of kiss butt wishy-washy unadulterated crap statements. NOAA and Jim Hansen need to just go away so we can restore some sanity to this Alice in Wonderland world.

  3. Posted April 11, 2009 at 11:08 am | Permalink


    Almost the whole ‘problem’ is that Jim Hansen has decided to pretty much dispense with hedging language and weasel words when the data – or his expert opinion – says the confidence is getting pretty high (but not certain) that the future risks will be socially devastating.

    As a journalist, you have to spot this language ‘use/misuse’ and avoid yourself turning it into fuel for deniers.

  4. Posted April 11, 2009 at 11:35 am | Permalink

    “Almost the whole ‘problem’ is that Jim Hansen has decided to pretty much dispense with hedging language and weasel words when the data – or his expert opinion – says the confidence is getting pretty high (but not certain) that the future risks will be socially devastating.”

    To be fair, I would agree with Tom that Hansen is moving beyond the “data” and his “expert” area. For example, Hansen apparently believes that sea level rise of several meters this century is a realistic possibility if CO2 levels continue to rise. But most experts in this area are now projecting likely sea level rise at around one meter per century (already up from the IPCC AR4 projections). No scientist, is above reasonable criticism, not even James Hansen, even if he has done more than any other scientist to alert the world to the problem of anthropogenic global warming.

    However, I do agree with Leonard that it is better not to focus so much on possible exaggerations or disagreement, which only provide “denialist” talking points.

    A more balanced report might focus more on Hansen’s strengths. He has made enormous contributions in assessing global temperature (NASA GISSTemp) and modeling future climate change. For example, Hansen’s scenario B has held up pretty well, all things considered.

    Reading the post, the call from the scientific community for strong urgent action is very much blunted by dwelling on the various points of disagreement. We can’t quantify the likely devastation, but devastation there will be if we continue on. After all, no one should be relieved to learn that sea level will be “only” one meter by 2100.

  5. Steve Bloom
    Posted April 11, 2009 at 2:55 pm | Permalink

    What Leonard said. It seems that Tom caught what he went fishing for.

    I do wonder how familiar Tans and Hoerling are with the “Target CO2″ paper. Tans isn’t, since 350 ppm is not identified as a tipping point (rather it’s a hopefully safe level substantially below the range likely to include the tipping point).

    Hoerling isn’t as well. It’s easy to miss the reference in “Target CO2,” but there very much is a firm scientific basis for thinking sea level might rise several meters in the next century. Hansen considers that BAU will probably involve much greater forcing applied to the ice sheets than was the case at the comparable stage in the Eemian, when the rise could have been as large as 2.6 meters/century (paper). Abstract:

    “The last interglacial period, Marine Isotope Stage (MIS) 5e, was characterized by global mean surface temperatures that were at least 2C warmer than present. Mean sea level stood 4–6m higher than modern sea level, with an important contribution from a reduction of the Greenland ice sheet. Although some fossil reef data indicate sea-level fluctuations of up to 10m around the mean, so far it has not been possible to constrain the duration and rates of change of these shorter-term variations. Here, we use a combination of a continuous high-resolution sea level record, based on the stable oxygen isotopes of planktonic foraminifera from the central Red Sea, and age constraints from coral data to estimate rates of sea-level change during MIS-5e. We find average rates of sea-level rise of 1.6m per century. As global mean temperatures during MIS-5e were comparable to projections for future climate change under the influence of anthropogenic greenhouse-gas emissions, these observed rates of sea-level change inform the ongoing debate about high versus low rates of sea-level rise in the coming century.

    DC, my impression is that the one meter is now more or less a consensus minimum.

    I don’t know what Hoerling is talking about re the Western snow pack, but at the very least he’s got an argument from these folks (co-authors include Tim Barnett, Ben Santer and Dan Cayan). Abstract:

    “Observations have shown the hydrological cycle of the western U.S. changed significantly over the last half of the twentieth century. Here we present a regional, multivariable climate-change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show up to 60% of the climate related trends of river flow, winter air temperature and snow pack between 1950-1999 are human-induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.”

  6. Posted April 11, 2009 at 4:48 pm | Permalink


    I probably should have said 1-1.5 m. I do agree that no one expects it to be below 1 m under BAU assumptions or even moderate mitigation scenarios.

    I went back to the realclimate.org discussion of Grinsted, Moore and Jevrejeva published in Climate Dynamics where they give 1.3 m for the A1F1 scenario. Stefan Rahmsdorf was in similar territory, but with *very* different equilibrium assumptions.


    As Grinstead said:

    “…I want to emphasize the agreement between our projections. We both project sea level in the order of a meter or more by the end of this century. That is considerably more than the IPCC AR4.”

    I hope we can both agree the main point should be that IPCC AR4 projections in this and other areas look already to be way too conservative, and that although Hansen may represent the upper limit of the range, projections of SLR as a whole are definitely increasing.

    Since you seem to have these facts at your fingertips (seriously, my impression is you may be the smartest guy in the room here), maybe you can elaborate a bit on Hansen’s SLR projections. My understanding is that Hansen projects up to 7m SLR by 2100. And are there other researchers who are in that range?

    I’m willing to reconsider or refine my position. On the other hand I’m already scared stiff as it is.

  7. Posted April 11, 2009 at 5:24 pm | Permalink

    To be honest, I’m pretty scared too.

    One of the problems with my post is that it represents a small fraction of all the information I gathered over the course of two days, and I struggled to find both a focus and the right balance of information. Although my interviews convinced me that Hansen is a bit out on a limb, I do believe the criticism of my post is fair.

    I will be adding video of an interview of Hansen that I conducted with my friend and colleague Susan Moran. In the video, Hansen will speak for himself. I’m hoping to get it posted by Monday.

  8. Steve Bloom
    Posted April 11, 2009 at 5:49 pm | Permalink

    DC, to my knowledge Hansen hasn’t gone further than “several meters” based on the rationale I described above. IIRC he did do a hand-wavy projection at one point coming up with a larger number, I think based on averaging end-glaciation SLR rates, but that was just to make a rhetorical point about the hyper-conservative nature of the IPCC projection. I’m not aware of anyone else going any farther in public, although who knows what dark thoughts might be shared over post-conference beer. As you say, one meter is already pretty bad.

    I’m flattered by your characterization of me, BTW, but I’m just an amateur who tries to pay close attention to the literature. With only limited success, I might add, since there’s so damned much of it.

  9. Steve Bloom
    Posted April 12, 2009 at 4:14 pm | Permalink

    Correction: Hansen’s rhetorical point was based on a doubling of current SLR every ten years. This gives a large result for 2100, which he stated was likely not orrect but was probably closer to reality than the IPCC estimate.

  10. Richard Ordway
    Posted May 28, 2009 at 8:33 pm | Permalink

    Wow, a blog that actually seems to discuss science.

    Anyway, I have talked privately to some IPCC lead authors and other IPCC gents (I haven’t talked with the women much) and gotten some of their deepest fears (I’ve known some for 11 or so years and shared my own concerns) to some of them. These blog’s words are often conservative compared to what they privately told me…you really should talk to them at their deepest level.

    From my conversations, some are really scared from what they told me privately. They portrayed some horrific and unthinkable possible future living conditions in the United States if we continue on our current path of carbon emissions.

    However, all that I have talked to so far, state that they personally think that there is still time (“barely”) left to act.

    They are literally some of the world’s most knowlegeable, most experienced and most refereed world’s climate scientists…and publcily (and responsibly) they are not saying what they told me privately.

    I was at NCAR for 11 years (you can still look me up on the NCAR website).

    Remember, they also said that they think that there is still time left to act.


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