Arguments
Study offers clues on 20th century global warming wobbles
Posted on 2 September 2013 by dana1981
This is a re-post of an article on The Conversation by Sunanda Creagh and Merran Reed with an interview of John Cook
The amount of solar radiation passing through Earth’s atmosphere and reaching the ground globally peaked in the 1930s, substantially decreased from the 1940s to the 1970s, and changed little after that, a new study has found.
The study, published in the journal Proceedings of the National Academy of Sciences, found that “neither the rapid increase in temperature from the 1970s through the 1990s nor the slowdown of warming in the early 21st century appear to be significantly related to changes of Rs (solar radiation reaching the Earth’s surface)”.
The new finding may help explain variations in warming during the 20th century. The authors showed that, while aerosols and clouds did play some role in temperature variations, they did not have a major effect on global mean land temperatures after 1985.
The authors, Kaicun Wang from Beijing Normal University and Robert E. Dickinson from the University of Texas at Austin, compiled a global data set of daily temperatures from the 1900s and through to 2010.
They analysed the relationship between the amount of solar radiation reaching the Earth and diurnal temperature range (the daily temperature variations that occur as day turns into night).
The authors of the study said that “the overall increase of global temperature over the last century has been largely attributable to the increase of greenhouse gases. Less well understood are the reasons for the variability of this increase on a decadal time scale… However, global temperatures do not appear to be significantly affected by changing Rs (solar radiation reaching the Earth’s surface).”
Wobbles in warming
Steve Sherwood, Director of the Climate Change Research Centre at the University of New South Wales, said the new paper was not really about whether the sun drives climate change.
“We already know from direct observations of the power coming from the sun that it has contributed nothing to global warming since 1979, though it probably made a small contribution to warming early in the 20th century,” said Professor Sherwood, who was not involved in the study.
“What this paper is really about is trying to explain the wobbles along the way in warming during the 20th century, and in particular the hiatus from about 1940 to 1970 in global warming, which was followed by strong warming thereafter. There has been a long debate as to whether such wobbles have been due to natural variations in ocean heat uptake, or to variations in aerosols (or clouds),” he said.
“This paper shows that aerosols and clouds did play some role but have never altered the global-mean land temperature by more than 0.1 to 0.2 degrees, while its overall warming has been over 1 degree.”
John Cook, Climate Communication Research Fellow at the University of Queensland, said there was growing evidence that solar activity has made little to no contribution to global warming over recent decades.
“In fact, several recent studies have found the sun and climate have been moving in opposite directions, with the sun having a slight cooling effect,” said Cook, who was not involved in the new study.
The paper's URL.
The headline numbers for climate-watchers will be the impact of Rs on Global Land Temperatures shown in Fig 6a & also presented in Table 1 as rate of change per century yielding on the back of my envelope - 1900-39 +0.02C, 1940-84 -0.16C, 1985-2010 +0.017C, with this last period starting in the aftermath of the El Chichon eruption.
Interesting paper with a different approach to a long-standing problem. Well worth a read, though some aspects are certainly debatable. One thing in the paper that I tend to disagree with is the authors notion that aerosol effects are likely less over oceans. Given the low albedo, even a small increase in aerosol the loading matters (more than over land). To their credit, the SH oceans are indeed less affected, although this might have changed in the last decade. All effects together, I don't think they overestimate the global temperature trends due to aerosols with their land only results.
Seems like there is a growing consensus for ocean cycles being the dominant factor in atmospheric warming "wobbles" rather than aerosols or (even further out of the running) solar variation. What's ironic is that the same deniers who long insisted that global warming itself was simply the result of these ocean cycles now refuse to believe that they could possibly be responsible for decreased atmospheric warming.
@CBDunkerson:
I'm afraid I have to disagree ;). While this paper is a promising attempt to narrow down the attributable temperature range (of individual forcers/internal variability), I would be cautious to draw definite conclusion regarding the total aerosol effect. Their proposed direct DTR-solar radiation link works well if only one forcing agent is involved, but other forcing agents do contribute as well, particularly when it comes to cloud feedbacks. Aerosols are certainly the major player, but the results are far from robust enough to overturn the huge amount of literature which points to stronger aerosol effcts. In fact, the "ocean cycle school" has been considerably challenged in recent years by the "external forcing school". One paper won't change that trend ;)
K.a.r.S.t.e.N, I agree that one paper doesn't change things, but I have been seeing a lot of papers pushing ocean cycles lately and nothing on aerosols for quite some time. Hence my perception of a shift in the balance of thought on the subject. It is possible that the shift has just been in the papers I have happened to encounter, but that would be a rather odd series of coincidences.
On cloud feedbacks... it wouldn't surprise me if aerosols play a greater role there than ocean cycles, but the general trend of the research I've seen has suggested that cloud feedbacks are likely small (unless that too is an incomplete picture).
@CBDunkerson:
The ocean is responsible for that what is happening right now. Anthropogenic aerosols play a minor role in the last 10-30 years. No contradiction here. Some volcanic (stratospheric) background aerosol played a role in the last decade, but that's about it (apart from solar variability of course).
With respect to the mid-century hiatus, I am not aware of any paper which managed to minimize the aerosol impact in a convincing fashion. The paper in question here is certainly argueing for bit lower an effect, but the problem what I see is that models do produce stronger aerosol effects even in the absence of a strongly reduced DTR. Models with better aerosol representation get closer to the observed DTR, but still, the global temperature response is stronger than suggested in this paper. Given the potential limitations of their study, this isn't enough to reject the current wisdom.
Cloud lifetime effects are believed to be small indeed. However, the cloud albedo effect is fairly significant and commonly believed to be stronger than the direct effect. While it also reduces the incoming solar radiation, I am not sure whether the DTR effect would be the same. Zhou et al. 2010 elaborated on that. Apart from that, a huge amount of aerosol papers kept coming in, without indications of a strongly reduced aerosol effect. The most compelling effort was published by Wilcox et al. 2013 earlier this year. Ellie Highwood has a neat blog post about it: elliehighwood.wordpress.com
Highly recommended read ;). Otherwise, please keep that image in mind:
The sulfate aerosol forcing might have triggered some ocean response which acted to amplify the initial forcing (e.g. enhanced interhemispheric heat transport) - there is indeed some evidence for that - but the main culprit can be seen above ... as far as my opinion is concerned ;)
For what it is worth, I think the location at which the aerosols are emitted may matter a lot. We know, for example, that the Earth is particularly sensitive to fluctuations in temperature in the Eastern Tropical Pacific. That being the case, if the aerosol emissions had come from the Galapagos Island, it is likely that they would have had a stronger effect than equivalent emissions in Australia. As it happens, the Northern Hemisphere in particular, is also sensitive to events in the North Atlantic. That suggests that aerosol emissions in the Eastern US, and Europe may have a stronger effect than emissions in India or China. Given the highly regional nature of the effect of aerosol emissions, it is at least unsafe to assume that their global effect will be the same regardless of where emited.
Of course, I am aware of no papers along this line, so this is just rampant speculation.
Tom, there was an interesting paper published earlier this year by Daniel Murphy, argueing that the recent redistribution of anthropogenic aerosols towards the equator didn't change the overall forcing picture much (under the assumption that the aerosol loading and composition remains constant). Locally things do change, but globally there is surprisingly little change. Seems to be consistent with another paper just published by Xie et al., showing that spatial patterns of GHG and aerosol response are similar, despite the heterogeneous aerosol forcing pattern.
While it might be a valid assumption for most purposes, that doesn't mean that it is safe to assume that their global effect will be the same regardless of where emitted. I absolutely agree with you here. Aerosols in formerly pristine regions have a considerable stronger effect, and indeed, the NH forcing should be very sensitive to the North Atlantic (and perhaps even ENSO), while this might not be such serious an issue elsewhere.
Re my first comment (#2), it might be worth adding Fig. 15 from Shindell et al. 2013, showing how strong an effect the indirect aerosol effect (mainly cloud albedo effect) has over the oceans (at least in the model world ;-)):