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Paleoclimate: The End of the Holocene

Recently a group of researchers from Harvard and Oregon State University has published the first global temperature reconstruction for the last 11,000 years – that’s the whole Holocene (Marcott et al. 2013). The results are striking and worthy of further discussion, after the authors have already commented on their results in this blog.
A while ago, I discussed here the new, comprehensive climate reconstruction from the PAGES 2k project for the past 2000 years. But what came before that? Does the long-term cooling trend that ruled most of the last two millennia reach even further back into the past?

Over the last decades, numerous researchers have painstakingly collected, analyzed, dated, and calibrated many data series that allow us to reconstruct climate before the age of direct measurements. Such data come e.g. from sediment drilling in the deep sea, from corals, ice cores and other sources. Shaun Marcott and colleagues for the first time assembled 73 such data sets from around the world into a global temperature reconstruction for the Holocene, published in Science. Or strictly speaking, many such reconstructions: they have tried about twenty different averaging methods and also carried out 1,000 Monte Carlo simulations with random errors added to the dating of the individual data series to demonstrate the robustness of their results.

To show the main result straight away, it looks like this:

Figure 1 Blue curve: Global temperature reconstruction from proxy data of Marcott et al, Science 2013. Shown here is the RegEM version – significant differences between the variants with different averaging methods arise only towards the end, where the number of proxy series decreases. This does not matter since the recent temperature evolution is well known from instrumental measurements, shown in red (global temperature from the instrumental HadCRU data). Graph: Klaus Bitterman.

The climate curve looks like a “hump”. At the beginning of the Holocene – after the end of the last Ice Age – global temperature increased, and subsequently it decreased again by 0.7 ° C over the past 5000 years. The well-known transition from the relatively warm Medieval into the “little ice age” turns out to be part of a much longer-term cooling, which ended abruptly with the rapid warming of the 20th Century. Within a hundred years, the cooling of the previous 5000 years was undone. (One result of this is, for example, that the famous iceman ‘Ötzi’, who disappeared under ice 5000 years ago, reappeared in 1991.)

The shape of the curve is probably not surprising to climate scientists as it fits with the forcing due to orbital cycles. Marcott et al. illustrate the orbital forcing with this graphic:

Figure 2 Changes in incoming solar radiation as a function of latitude in December, January and annual average, due to the astronomical Milankovitch cycles (known as orbital forcing). Source: Marcott et al., 2013.

In the bottom panel we see the sunlight averaged over the year, as it depends on time and latitude. It declined strongly in the mid to high latitudes over the Holocene, but increased slightly in the tropics. In the Marcott reconstruction the global temperature curve is dominated primarily by the large temperature changes in northern latitudes (30-90 °N). For this, the middle panel is particularly relevant: the summer maximum of the incoming radiation. That reduces massively during the Holocene – by more than 30 watts per square meter. (For comparison: the anthropogenic carbon dioxide in the atmosphere produces a radiative forcing of about 2 watts per square meter – albeit globally and throughout the year.) The climate system is particularly sensitive to this summer insolation, because it is amplified by the snow- and ice-albedo feedback. That is why in the Milanković theory summer insolation is the determining factor for the ice age cycles – the strong radiation maximum at the beginning of the Holocene is the reason why the ice masses of the last Ice Age disappeared.

However a puzzle remains: climate models don’t seem to get this cooling trend over the last 5,000 years. Maybe they are underestimating the feedbacks that amplify the northern orbital forcing shown in Fig. 2. Or maybe the proxy data do not properly represent the annual mean temperature but have a summer bias – as Fig. 2 shows, it is in summer that the solar radiation has declined so strongly since the mid-Holocene. As Gavin has just explained very nicely: a model-data mismatch is an opportunity to learn something new, but it takes work to find out what it is.

Comparison with the PAGES 2k reconstruction

The data used by Marcott et al. are different from those of the PAGES 2k project (which used land data only) mainly in that they come to 80% from deep-sea sediments. Sediments reach further back in time (far further than just through the Holocene – but that’s another story). Unlike tree-ring data, which are mainly suitable for the last two thousand years and rarely reach further. However, the sediment data have poorer time resolution and do not extend right up to the present, because the surface of the sediment is disturbed when the sediment core is taken. The methods of temperature reconstruction are very different from those used with the land data. For example, in sediment data the concentration of oxygen isotopes or the ratio of magnesium to calcium in the calcite shells of microscopic plankton are used, both of which show a good correlation with the water temperature. Thus each sediment core can be individually calibrated to obtain a temperature time series for each location.

Overall, the new Marcott reconstruction is largely independent of, and nicely complementary to, the PAGES 2k reconstruction: ocean instead of land, completely different methodology. Therefore, a comparison between the two is interesting:

Figure 3 The last two thousand years from Figure 1, in comparison to the PAGES 2k reconstruction (green), which was recently described here in detail. Graph: Klaus Bitterman.

As we can see, both reconstruction methods give consistent results. That the evolution of the last one thousand years is virtually identical is, by the way, yet another confirmation of the “hockey stick” by Mann et al. 1999, which is practically identical as well (see graph in my PAGES article).

Is the modern warming unique?

Because of the above-mentioned limitations of sediment cores, the new reconstruction does not reach the present but only goes to 1940, and the number of data curves used already strongly declines before that. (Hence we see the uncertainty range getting wider towards the end and the reconstructions with different averaging methods diverge there – we here show the RegEM method because it deals best with the decreasing data coverage. For a detailed analysis see the article by statistician Grant Foster.) The warming of the 20th Century can only be seen partially – but this is not serious, because this warming is very well documented by weather stations anyway. There can be no doubt about the climatic warming during the 20th Century.

There is a degree of flexibility on how the proxy data (blue) should be joined with the thermometer data (red) – here I’ve done this so that for the period 1000 to 1940 AD the average temperature of the Marcott curve and the PAGES 2k reconstruction are equal. I think this is better than the choice of Marcott et al. (whose paper was published before PAGES 2k) – but this is not important. The relative positioning of the curves makes a difference for whether the temperatures are slightly higher at the end than ever before in the Holocene, or only (as Marcott et al write) higher than during 85% of the Holocene. Let us just say they are roughly as high as during the Holocene optimum: maybe slightly cooler, maybe slightly warmer. This is not critical.

The important point is that the rapid rise in the 20th Century is unique throughout the Holocene. Whether this really is true has been intensively discussed in the blogs after the publication of the Marcott paper. Because the proxy data have only a coarse time resolution – would they have shown it if there had been a similarly rapid warming earlier in the Holocene?

I think for three reasons it is extremely likely that there was not such a rapid warming before:

1. There are a number of high-resolution proxy data series over the Holocene, none of which suggest that there was a previous warming spike as strong as in the 20th Century. Had there been such a global warming before, it would very likely have registered clearly in some of these data series, even if it didn’t show up in the averaged Marcott curve.

2. Grant Foster performed the test and hid some “20th C style” heating spikes in earlier parts of the proxy data to see whether they are revealed by the method of Marcott et al – the answer is a resounding yes, they would show up (albeit attenuated) in the averaged curve, see his article if you are interested in the details.

3. Such heating must have a physical basis, and it would have to have quickly disappeared again (would it have lasted, it would be even more evident in the proxy data). There is no evidence in the forcing data that such a climate forcing could have suddenly appeared and disappeared, and I cannot imagine what could have been the mechanism. (A CO2-induced warming would persist until the CO2 concentration decays again over thousands of years – and of course we have good data on the concentration of CO2 and other greenhouse gases for the whole Holocene.)

Conclusion

The curve (or better curves) of Marcott et al. will not be the last word on the global temperature history during the Holocene; like Mann et al. in 1998 it is the opening of the scientific discussion. There will certainly be alternative proposals, and here and there some corrections and improvements. However, I believe that (as was the case with Mann et al. for the last millennium) the basic shape will turn out to be robust: a relatively smooth curve with slow cooling trend lasting millennia from the Holocene optimum to the “little ice age”, mainly driven by the orbital cycles. At the end this cooling trend is abruptly reversed by the modern anthropogenic warming.

The following graph shows the Marcott reconstruction complemented by some context: the warming at the end of the last Ice Age (which 20,000 years ago reached its peak) and a medium projection for the expected warming in the 21st Century if humanity does not quickly reduce greenhouse gas emissions.

Figure 4 Global temperature variation since the last ice age 20,000 years ago, extended until 2100 for a medium emissions scenario with about 3 degrees of global warming. Graph: Jos Hagelaars.

Marcott et al. dryly state about this future prospect:

By 2100, global average temperatures will probably be 5 to 12 standard deviations above the Holocene temperature mean.

In other words: We are catapulting ourselves way out of the Holocene.

Just looking at the known drivers (climate forcings) and the actual temperature history shows it directly, without need for a climate model: without the increase in greenhouse gases caused by humans, the slow cooling trend would have continued. Thus virtually the entire warming of the 20th Century is due to man. This May, for the first time in at least a million years, the concentration of carbon dioxide in our atmosphere has exceeded the threshold of 400 ppm.

Energy

Natural Gas May Be Easier On Climate Than Coal, Despite Methane Leaks

Enlarge image

Credit Brennan Linsley / AP

A rig drills a hydraulic fracturing well for natural gas outside Rifle, Colo., in March.

Originally published on Mon September 16, 2013 6:11 pm

From the standpoint of global warming, burning natural gas can be better than burning coal, a study published this week suggests.

This is a contentious issue among people who are opposed to the natural gas drilling practice known as fracking. That technique involves injecting water, sand and chemicals into wells to release far more gas than conventional drilling can. Opponents of fracking have been concerned not only about local environmental issues, but also about the potential for methane leaks to make global warming worse.

Even though natural gas burns much cleaner than coal, the main constituent, methane, also leaks into the atmosphere during production. Methane is a potent greenhouse gas, so those leaks could potentially wipe out the climate benefits of natural gas.

And fracking technology took off before anyone really understood how much natural gas leaks out in the process.

“We wanted to go out and collect some of the first data on some of the new types of operations underway in natural gas production and what the methane emissions are,” says David Allen, an engineering professor at the University of Texas in Austin.

Allen got funding from the Environmental Defense Fund, as well as support from nine major companies that volunteered to participate in the study. His conclusion: Currently the U.S. Environmental Protection Agency greatly overestimates methane emissions from a new well that is being prepared to produce gas for the first time. But he found that the EPA also greatly underestimates emissions from wells that are already in production. And when you add the whole thing up, it’s basically a wash, Allen says.

That’s potentially good news for advocates of natural gas, because it supports the argument that, if done right, natural gas production can be much better for the climate than coal.

The study appears in the Proceedings of the National Academy of Sciences, and suggests ways to make the production of natural gas cleaner than it is today.

For example, it turns out there’s a lot of methane leakage from fracking operations that separate natural gas from oil and water as they come up the well at the same time.

“That was the biggest surprise for me, as an operator,” says Edwin Hance from Pioneer Natural Resources, a natural gas company that operates mostly in Texas and participated in the study.

“I would say that’s the primary area we need to focus going forward,” Hance says, “as far as practices where we can have the opportunity to reduce emissions further.”

It’s important to note that the study relied on data from nine companies, all of which volunteered to be studied. Bob Howarth, a professor of ecology and environmental biology at Cornell University, says that means the results from the 190 study sites are not necessarily representative of the industry as a whole.

“I would view this as best-case scenario for what industry can do to reduce methane emissions when they want to,” Howarth says.

A different study, published last month by the National Oceanic and Atmospheric Administration, may be more representative of a worst-case scenario. The NOAA study sampled the air in an entire basin in Utah, rather than tracking emissions down to specific pieces of equipment and specific practices.

“They’re finding methane emissions that are 10 to 20 times higher than this new study,” Howarth says, “and I think [that’s] probably more representative of at least those western gas fields, when industry does not realize it’s being watched.”

Figuring out what practices are responsible for large methane emissions, like those in Utah, will take more work.

Steve Hamburg, chief scientist at the Environmental Defense Fund, says his organization is funding 16 studies to look at the entire natural gas system in the United States. The PNAS study, focusing only on production, is just one part of that.

“Regrettably, we need another year, and then we’ll have all of these pieces together and we really can get a much clearer picture of what’s going on,” Hamburg says.

At stake isn’t simply gas production in the United States. Natural gas is taking off globally. So Hamburg says these measurements offer producers and regulators an opportunity to fix what’s wrong in the U.S. and to spread those best practices around

• 
• • Graham Readfearn
  • theguardian.com, Tuesday 17 September 2013 09.49 AEST
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Climate change has become a political football in Australia in a game with few rules and, depending on your chosen sporting code, plenty of illegal spear tackling and studs-up challenges.

To continue with our sporting analogy, Australia’s prime minister elect Tony Abbott has just taken the climate change ball and locked it in a cupboard at the back of the official residence. And in the same cupboard, there’s a fancy-looking box with the word “science” written on it (there is some awesome stuff in there, including iPhones, advances in agriculture and the occasional medical breakthrough).

Abbott’s first cabinet, announced yesterday, has a front bench team comprised of ministers for defence, immigration and border protection, the arts, agriculture, health and sport and small business, among others. Yet there will be no minister for science. Abbott suggested to reporters this would come under the portfolio of Ian Macfarlane, who was yesterday named minister for industry. In opposition, Greg Hunt’s position was the shadow minister for climate action, environment and heritage. Abbott has taken his eraser to Hunt’s title, rubbing out the words “climate action” and “heritage”.

Science didn’t exactly stick out like a Higgs Boson in the previous government’s ministry either. Senator Kim Carr was officially the minister for innovation, industry, science and research in a department with a name so long it could be barely uttered in a single breath (OK – I exaggerate, it was the department of industry, innovation, climate change, science, research and tertiary education).

One Liberal MP disappointed with the lack of a science portfolio would be the member for Tangney, Dennis Jensen, who told the media last week he was the right person for the job-that-wasn’t. Jensen also had a few thoughts about climate change and my questions to him about his partial-endorsement of climate sceptic Lord Christopher Monckton remain unanswered.

Jensen won’t be alone in bemoaning the lack of an obvious portfolio for science. Around 68,000 “scientists and technologists” are also scratching their heads waiting for answers, according to Catriona Jackson, the chief executive of Science & Technology Australia, the body which represents them.

She said earlier: “Scientists around the nation are asking, where is the science minister?’” She told me the nation’s scientists would be “confused and disappointed” by the absence of an obvious ministerial position to represent a sector which, she said, was “at the heart of everything that government does”. She added that Abbott’s plans for science will be clearer when the detailed administrative arrangements are released “any day now”, but we might presume Macfarlane’s portfolio will also include energy (another key word missing from the list of titles), giving the new industry minister a role very similar to that which he played under the government of John Howard.

Back in 2006, Macfarlane was named by academic, author and public intellectual Clive Hamilton as being amongst a “dirty dozen” of Australians characterised as the “greenhouse mafia” who had worked harder than any others to “prevent any effective action to reduce Australia’s burgeoning greenhouse gas emissions”. In an interview with the ABC in 2007, Macfarlane was pressed on his views about human-caused climate change and was unconvincing in his response.

Thankfully since then, the implications for policy makers from the science on human-caused climate change have become clearer (not that they weren’t clear back then). It’s happening, it’s us and we should take it really seriously. The fossil fuel industry will be hoping that Macfarlane’s views haven’t changed.

Hunt has always insisted that human-caused climate change is a serious issue. Now he just needs to go and ask the boss if he can get the ball out of the cupboard and do something meaningful with it. The signs are not good.

• The 5 stages of climate denial are on display ahead of the IPCC report

  Rupert Murdoch’s Wall Street Journal and The Australian are providing the media coverage for climate contrarian damage control. Photograph: Stefan Rousseau/PA

  Dana Nuccitelli: Climate contrarians appear to be running damage control in the media before the next IPCC report is published

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  12.42 EST

• Wednesday 11 September 2013

• Global warming, ethanol, and will-o-wisp solutions

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  John Abraham: Contrary to common perceptions, corn-based ethanol is a useful technology on the road to reducing carbon emissions

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• Monday 9 September 2013

• Arctic sea ice delusions strike the Mail on Sunday and Telegraph

  It’s only a matter of time before we experience an Arctic sea ice-free summer. Photograph: Jenny E Ross/Corbis

  Dana Nuccitelli: Both UK periodicals focus on short-term noise and ignore the rapid long-term Arctic sea ice death spiral

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  13.26 EST

• Tuesday 3 September 2013

• The Pacific Ocean fills in another piece of the global warming puzzle

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  Dana Nuccitelli: Evidence continues to mount that the slowed warming of global surface temperatures is mainly due to changes in the oceans

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• Friday 30 August 2013

• Now available: a guide to the IPCC’s new RCP emissions pathways

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  Graham Wayne: The new IPCC RCPs describe four scenarios how the planet might change in the future, for climate research and modeling

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• Wednesday 28 August 2013

• Global warming and oceans: what are the known unknowns?

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• Friday 23 August 2013

• Is climate change humanity’s greatest-ever risk management failure?

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• Wednesday 21 August 2013

• How extreme will future heatwaves be? Choose your own adventure

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  Dana Nuccitelli: Depending on our greenhouse gas emissions path, today’s extreme heat could become the norm, or could be relatively rare

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  14.28 EST

• Monday 19 August 2013

• With the forthcoming IPCC report, the contrarians finally agree we are changing the climate

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  John Abraham: Climate contrarians may concede more than they bargained for when the next IPCC report is published

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• Wednesday 14 August 2013

• A grand solar minimum would barely make a dent in human-caused global warming

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  Dana Nuccitelli: Research has shown that a grand solar minimum would offset no more than 0.3°C of global warming

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  14.52 EST

• Monday 12 August 2013

• Global warming, Arctic ice loss, and armchair scientists

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  John Abraham: Armchair scientist Neven provides valuable insights into the rapid decline of Arctic sea ice

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  16.11 EST

• Thursday 8 August 2013

• Fox News found to be a major driving force behind global warming denial

  Fox News is helping prop up conservative American global warming denial.

  Dana Nuccitelli: American consumers of conservative media like Fox News distrust climate scientists and don’t believe the planet is warming

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• Tuesday 6 August 2013

• For some climate scientists, speaking out is a moral obligation

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• Friday 2 August 2013

• It’s climate scientists, not concern trolls who champion the scientific method

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  Dana Nuccitelli: Warren Pearce has mistaken concern trolling for caring about the scientific method

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  14.42 EST

• Tuesday 30 July 2013

• Can a carbon tax work without hurting the economy? Ask British Columbia

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  Dana Nuccitelli: Five years in, BC’s carbon tax has successfully reduced greenhouse gas emissions in a stable economy

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