PBMC - Painel Brasileiro de Mudanças Climáticas

Memo: Examining the Recent "Pause" in Global Warming

"Global surface temperatures have warmed more slowly over the past decade than previously expected. The media has seized this warming pause in recent weeks, and the UK’s Met Office released a three-part series of white papers looking at the causes and implications. While there is still no definitive cause identified, some researchers point to a combination of more heat going into the deep oceans and downturns in multi-decadal cycles in global temperature as the primary drivers of the pause. Others argue that a plethora of recent small volcanoes, changes in stratospheric water vapor, and a downturn in solar energy reaching the earth may also be contributing to the plateau. While few expect the pause to persist much longer, it has raised some questions about the growing divergence between observed temperatures and those predicted by climate models."                     Read more...

A Pause, Not an End, to Warming

By RICHARD A. MULLER Published: September 25, 2013

BERKELEY, Calif. — THE global warming crowd has a problem. For all of its warnings, and despite a steady escalation of greenhouse gas emissions into the atmosphere, the planet’s average surface temperature has remained pretty much the same for the last 15 years.

As you might guess, skeptics of warming were in full attack mode as the Intergovernmental Panel on Climate Change gathered in Sweden this week to approve its latest findings about our warming planet. The skeptics argue that this recent plateau illustrates what they always knew — that complex global climate models have no predictive capability and that, therefore, there is no proof of global warming, human-caused or not.

Greenhouse theorists appear to be on the defensive as they offer different explanations for the letup — that deep ocean water may be draining some warmth from the atmosphere, that increases in high-altitude water vapor may be responsible or that numerous small volcanic eruptions are the cause.

My analysis is different. Berkeley Earth, a team of scientists I helped establish, found that the average land temperature had risen 1.5 degrees Celsius over the past 250 years. Solar variability didn’t match the pattern; greenhouse gases did.

As for the recent plateau, I predicted it, back in 2004. Well, not exactly. In an essay published online then at MIT Technology Review, I worried that the famous “hockey stick” graph plotted by three American climatologists in the late 1990s portrayed the global warming curve with too much certainty and inappropriate simplicity. The graph shows a long, relatively unwavering line of temperatures across the last millennium (the stick), followed by a sharp, upward turn of warming over the last century (the blade). The upward turn implied that greenhouse gases had become so dominant that future temperatures would rise well above their variability and closely track carbon dioxide levels in the atmosphere.

I knew that wasn’t the case. The planet warmed by 0.6 degrees over the prior 50 years, but occasional, unexplained temperature fluctuations of as much as 0.3 degrees countered the rise at times and resulted in apparent pauses. Some of the fluctuations might have been caused by shifting ocean currents related to the Gulf Stream and El Niño — the episodic appearance of unusually warm ocean temperatures along the west coast of South America. Here’s what I wrote in 2004:

“Suppose... future measurements in the years 2005-2015 show a clear and distinct global cooling trend. (It could happen.) If we mistakenly took the hockey stick seriously — that is, if we believed that natural fluctuations in climate are small — then we might conclude (mistakenly) that the cooling could not be just a random fluctuation on top of a long-term warming trend, since according to the hockey stick, such fluctuations are negligible. And that might lead in turn to the mistaken conclusion that global warming predictions are a lot of hooey. If, on the other hand, we reject the hockey stick, and recognize that natural fluctuations can be large, then we will not be misled by a few years of random cooling.”

O.K., I didn’t actually predict a pause in the warming but a possible period of cooling. But that’s close enough. We are now in that pause, and too many people are taking it too seriously, not just the skeptics and the media but even the greenhouse-warming advocates.

We don’t fully understand past variations, but there is a theorem in science: if it happens, it must be possible. The frequent rises and falls, virtually a stair-step pattern, are part of the historic record, and there is no expectation that they will stop, whatever their cause. A realistic prediction simply includes a similar variability as an unexplained component.

Of course, there are scientists who thought they had explained the variability. Previous pauses in temperature rise in 1982 and 1991 were attributed to the ash and sulfur aerosols spewed into the atmosphere by the volcanic eruptions of El Chichón in Mexico and Pinatubo in the Philippines, respectively. I never found those attributions compelling; in particular, the eruption of El Chichón was too small to account for the stall in warming that was attributed to it. I suspect it was more likely that the variations were the result of chaotic changes in ocean currents.

Because of the instability of ocean flow, the best evidence of a changing climate may be the land temperature record. It is full of fits and starts that make the upward trend vanish for short periods. Regardless of whether we understand them, there is no reason to expect them to stop. The best statistical test of an observation is to see if it has happened naturally in the past.

Most of us hope that global warming actually has stopped. (Not everyone; some argue that the warming is good.) Perhaps the negative feedback of cloud cover has kicked in, dampening global warming, or the ocean absorption of atmospheric heat is playing a new and more decisive role.

Alas, I think such optimism is premature. The current pause is consistent with numerous prior pauses. When walking up stairs in a tall building, it is a mistake to interpret a landing as the end of the climb. The slow rate of warming of the recent past is consistent with the kind of variability that some of us predicted nearly a decade ago. 

Fonte: The New York Times

Has Global Warming Stopped?

By Berkeley Earth Memo by Richard Muller updated 26 Sept 2013

In this memo I give my personal perspective on the widely discussed slowing of global warming over the past decade. My Op Ed on this subject appears in the New York Times on 26 Sept 2013. However, that Op Ed does not include the data plots that I find more compelling than a thousand words. 

The Berkeley Earth plot of land temperature change since 1950 is shown below. (For the full plot from 1753, see http://berkeleyearth.org/summary-of-findings.) The monthly data were smoothed with a one-year running average. The dark and light grey regions are the one and two standard deviation uncertainty estimates. The digital data to make this plot are available at http://berkeleyearth.lbl.gov/auto/Global/Full_TAVG_complete.txt 

The temperature curve above shows rapid up and down bumps that are strongly correlated with the Atlantic multidecadal oscillation and El Nino. For detailed discussion of the correlation, see our peer-reviewed and published papers, available online at http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50458/abstract  and http://www.scitechnol.com/2327-4581/2327-4581-1-101.pdf 

Now look at the temperature graph plotted above. The temperature has bounced around. The last few years appear to show an end to warming, even a drop. Nevertheless, I will show that this “pause” in global warming is statistically apocryphal. 

As aficionados of optical illusions know (and as all scientists have learned) eyeball impressions can be misleading. It is better to do a linear least-squares fit to the data. The plot below shows the results of picking some “interesting” time segments and doing such fits.

What you will note is that the temperature record is full of fits and spurts, starts and stops, with many segments that are well below the average slope of the global warming rise, or even negative – brief periods of cooling. From 2001 to the present (the data for the current century) the slope significantly reduced. This is the pause that gives skeptics joy and puts the global warming community on the defensive. 

But similar pauses and even more severe drops occur at several spots on the recent record. Any one of these, in the past, might have drawn attention as a slowing or reversal of warming. In fact, on June 24, 1974, Time Magazine noted the drop in temperature in an article “Another Ice Age?” Newsweek followed on April 28, 1975, with an article “The Cooling World”. Some of the pauses have been attributed to volcanic eruptions (Mt. Pinatubo erupted in June 1991) but the much larger swings are more closely associated with variations in ENSO, and the slower ones with the Atlantic Multidecadal Oscillation or AMO (again, see our paper http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50458/abstract). 

Note that the variations, the departures from the linear trend, have high statistical significance; the small error uncertainties (grey regions) indicate that the swings occur all around the world simultaneously. They are real – just not immediately related to the more gradual global warming trend. In fact, when we study the long-term trend, these variations are mathematically treated as noise, since they are not predictable even though they are real. I like to call this figure, with the line segment fits, the “stair step” plot. 

Is the recent pause statistically significant? Because of the ENSO and AMO variations, similar variations have occurred in the past. Are these the latest incarnations of ENSO/AMO? Without a fuller understanding of ENSO/AMO, we can’t be sure. Based on the record from 1970 to 2001, how likely is a pause similar to the one we see, based simply on the unrelated behavior o ENSO and AMO? Statistical significance is usually described as the likelihood that such a variation might occur given the past behavior of the data. In this case, since similar fluctuations are evident in the data, the current “pause” is not statistically significant. 

Bottom line: a look at the recent data gives the impression that global warming may have stopped. Maybe negative cloud cover feedback has kicked in! We can hope it has. But although such a pause may be occurring, as evaluated scientifically it has not yet achieved statistical significance.

Seminário: Lançamento do Sumário Executivo do GT2

Seminário: Lançamento do Sumário Executivo do GT2 - Primeiro Relatório de Avaliação Nacional do Painel Brasileiro de Mudanças Climáticas

25 de outubro de 2013, FBDS, Rio de Janeiro, RJ

Frente às interferências das ações humanas sobre o ambiente - que atingiram escala global e magnitude sem precedentes, afetando o funcionamento natural do sistema climático - os formuladores de políticas públicas e a sociedade em geral necessitam de informações objetivas sobre as causas das mudanças climáticas, seus impactos ambientais e socioeconômicos e as possíveis soluções.

Com base nisso, o Painel Brasileiro de Mudanças Climáticas (PBMC) foi estabelecido, nos moldes do Painel Intergovernamental de Mudanças Climáticas (IPCC, em inglês). O papel do PBMC é reunir, sintetizar e avaliar informações científicas sobre os aspectos relevantes das mudanças climáticas no Brasil. O Sumário Executivo do GT1 – Base Científica das Mudanças Climáticas, do Primeiro Relatório de Avaliação Nacional do Painel Brasileiro de Mudanças Climáticas foi lançado no dia 09 de setembro e está disponível para download.

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