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A new paper improves our estimate of the climate's sensitivity to carbon dioxide. NASA/Wikimedia Commons

One of the key questions about climate change is the strength of the greenhouse effect. In scientific terms this is described as “climate sensitivity”. It’s defined as the amount Earth’s average temperature will ultimately rise in response to a doubling of atmospheric carbon dioxide levels.

Climate sensitivity has been hard to pin down accurately. Climate models give a range of 1.5-4.5℃ per doubling of CO₂, whereas historical weather observations suggest a smaller range of 1.5-3.0℃ per doubling of CO₂.

In a new study published in Science Advances, Cristian Proistosescu and Peter J. Huybers of Harvard University resolve this discrepancy, by showing that the models are likely to be right.

According to their statistical analysis, historical weather observations reveal only a portion of the planet’s full response to rising CO₂ levels. The true climate sensitivity will only become manifest on a time scale of centuries, due to effects that researchers call “slow climate feedbacks”.

Fast and slow

To understand this, it is important to know precisely what we mean when we talk about climate sensitivity. So-called “equilibrium climate sensitivity”, or slow climate feedbacks, refers to the ultimate consequence of climate response – in other words, the final effects and environmental consequences that a given greenhouse gas concentration will deliver.

These can include long-term climate feedback processes such as ice sheet disintegration with consequent changes in Earth’s surface reflection (albedo), changes to vegetation patterns, and the release of greenhouse gases such as methane from soils, tundra or ocean sediments. These processes can take place on time scales of centuries or more. As such they can only be predicted using climate models based on prehistoric data and paleoclimate evidence.

On the other hand, when greenhouse gas forcing rises at a rate as high as 2–3 parts per million (ppm) of CO₂ per year, as is the case during the past decade or so, the rate of slow feedback processes may be accelerated.

Measurements of atmosphere and marine changes made since the Industrial Revolution (when humans first began the mass release of greenhouse gases) capture mainly the direct warming effects of CO₂, as well as short-term feedbacks such as changes to water vapour and clouds.

A study led by climatologist James Hansen concluded that climate sensitivity is about 3℃ for a doubling of CO₂ when considering only short-term feedbacks. However, it’s potentially as high as 6℃ when considering a final equilibrium involving much of the West and East Antarctic ice melting, if and when global greenhouse levels transcend the 500-700ppm CO₂ range.

This illustrates the problem with using historical weather observations to estimate climate sensitivity – it assumes the response will be linear. In fact, there are factors in the future that can push the curve upwards and increase climate variability, including transient reversals that might interrupt long-term warming. Put simply, temperatures have not yet caught up with the rising greenhouse gas levels.

Prehistoric climate records for the Holocene (10,000-250 years ago), the end of the last ice age roughly 11,700 years ago, and earlier periods such as the Eemian (around 115,000-130,000 years ago) suggest equilibrium climate sensitivities as high as 7.1-8.7℃.

So far we have experienced about 1.1℃ of average global warming since the Industrial Revolution. Over this time atmospheric CO₂ levels have risen from 280ppm to 410ppm – and the equivalent of more than 450ppm after factoring in the effects of all the other greenhouse gases besides CO₂.

Estimate of climate forcing for 1750-2000. Author provided Crossing the threshold

Climate change is unlikely to proceed in a linear way. Instead, there is a range of potential thresholds, tipping points, and points of no return that can be crossed during either warming or transient short-lived cooling pauses followed by further warming.

The prehistoric records of the cycles between ice ages, namely intervening warmer “interglacial” periods, reveal several such events, such as the big freeze that suddenly took hold about 12,900 years ago, and the abrupt thaw about 8,200 years ago.

In the prehistoric record, sudden freezing events (called “stadial events”) consistently follow peak interglacial temperatures.

Such events could include the collapse of the Atlantic Mid-Ocean Circulation (AMOC), with consequent widespread freezing associated with influx of extensive ice melt from the Greenland and other polar ice sheets. The influx of cold ice-melt water would abort the warm salt-rich AMOC, leading to regional cooling such as is recorded following each temperature peak during previous interglacial periods.

Over the past few years cold water pools south of Greenland have indicated such cooling of the North Atlantic Ocean. The current rate of global warming could potentially trigger the AMOC to collapse.

A collapse of the AMOC, which climate “sceptics” would no doubt welcome as “evidence of global cooling”, would represent a highly disruptive transient event that would damage agriculture, particularly in the Northern Hemisphere. Because of the cumulative build-up of greenhouse gases in the atmosphere such a cool pause is bound to be followed by resumed heating, consistent with IPCC projections.

The growth in the cold water region south of Greenland, heralding a possible collapse of the Atlantic Mid-Ocean Circulation. Author provided

Humanity’s release of greenhouse gases is unprecedented in speed and scale. But if we look far enough back in time we can get some clues as to what to expect. Around 56 million years ago, Earth experienced warming by 5-8℃ lasting several millennia, after a sudden release of methane-triggered feedbacks that caused the CO₂ level rise to around 1,800ppm.

Yet even that sudden rise of CO₂ levels was lower by a large factor than the current CO₂ rise rate of 2-3ppm per year. At this rate, unprecedented in Earth’s recorded history of the past 65 million years (with the exception of the consequences of asteroid impacts), the climate may be entering truly uncharted territory.

Andrew Glikson does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.

Egglestone, Teesdale For some, rain came as a blessed relief after days of drought and the downpour coaxed snails out to graze

The storm faded away to a distant rumble of thunder over the hills, taking with it the sticky heat of the past few days and leaving us shivering, in wet clothes, under a sheltering oak.

As we emerged, so did the insects. I watched a shield bug ease itself around the edge of a leaf back into the light, picking a path between wobbling water droplets. Spiders abseiled between grass stems, repairing webs. Within a few minutes bumblebees were at work again, shaking rain from water-laden bramble flowers.

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New data shows that energy consumers are paying more than they think in their bills, and more than what the regulator admits.

The latest guide to retail electricity prices across the major states in Australia.

This partnership will include a global distribution agreement, the inclusion of partner products in the respective portfolios, as well as joint research and development activities.

Energy Locals becomes second market player in week to hit out at fossil fuel generators for “deliberately” driving up electricity bills.

The European and Japanese space agencies put the finishing touches to their Mercury satellites.

Rising numbers of people want selfies riding elephants, or washing them, or patting their trunks, but according to a study carried out by World Animal Protection across Asia, many are kept in "severely cruel" conditions.

German-U.S. storage specialist Younicos snapped up by British power generation firm Aggreko in £40m deal.

The clean energy future looks unstoppable:  Volvo announces end of pure internal combustion engine cars - in two years.

The BBC's Jan Bruck has you covered.

Our clothing and laundry are polluting the marine environment, UK research reveals.

Our clothing and laundry are polluting the marine environment, UK research reveals.

Arrium buyer Sanjeev Gupta says key to future of Whyalla steel works is renewables energy, looking at pumped hydro, wind, solar and co-gen.

Sonnen offers grid power for homes with solar and battery storage for a fixed annual fee as low as $360. It says the mobile-phone style plan - that merges peer-to-peer trading and virtual power plants - is designed to take traditional utilities out of business.

PM will use summit meeting with US president to say climate change agreement doesn’t need renegotiation

Theresa May will raise the issue of climate change with Donald Trump this weekend when the pair meet for the first time since she lost her majority in the general election. They will talk at the G20 summit in Hamburg, which runs on Friday and Saturday.

The two leaders will hold a formal bilateral meeting, at which the prime minister plans to tell the US president she does not believe the Paris climate change agreement needs to be renegotiated.

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Icebergs breaking off Antarctica, even massive ones, do not typically concern glaciologists. But the impending birth of a new massive iceberg could be more than business as usual for the frozen continent.

The Larsen C ice shelf, the fourth-largest in Antarctica, has attracted worldwide attention in the lead-up to calving an iceberg one-tenth of its area – or about half the area of greater Melbourne. It is still difficult to predict exactly when it will break free.

But it’s not the size of the iceberg that should be getting attention. Icebergs calve all the time, including the occasional very large one, with nothing to worry about. Icebergs have only a tiny direct effect on sea level.

The calving itself will simply be the birth of another big iceberg. But there is valid concern among scientists that the entire Larsen C ice shelf could become unstable, and eventually break up entirely, with knock-on effects that could take decades to play out.

Ice shelves essentially act as corks in a bottle. Glaciers flow from land towards the sea, and their ice is eventually absorbed into the ice shelf. Removal of the ice shelf causes glaciers to flow faster, increasing the rate at which ice moves from the land into the sea. This has a much larger effect on sea level than iceberg calving does.

While the prediction that Larsen C could become unstable is based partly on physics, it is also based on observations. Using aerial and satellite images, scientists have been able to track very similar ice shelves in the past, some of which have been seen to retreat and collapse.

The death of an ice shelf

The most dramatic ice shelf collapse observed so far is that of Larsen C’s neighbour to the north – the imaginatively named Larsen B. Over the course of just six weeks in 2002 the entire ice shelf splintered into dozens of icebergs. Almost immediately afterwards, the glaciers feeding into it sped up by two to six times. Those glaciers continue to flow faster to this day.

Satellite photo series of Larsen B Ice Shelf collapse from January 2002 to April 2002. NASA

In our new study, published in Earth and Planetary Science Letters, we turn the clock back even further to look at the Wordie ice shelf, on the west coast of the southern Antarctic Peninsula, which began to retreat in the 1960s and eventually disappeared in January 2017.

Over the past 20 years, observations have shown that the main glacier feeding into the Wordie ice shelf, the Fleming Glacier, has sped up and thinned. Compared with the glaciers feeding Larsen B and C, Fleming Glacier is massive: 80km long, 12km wide, and 600m thick at its front.

Locations of the Larsen C Ice Shelf and the Wordie Ice Shelf-Fleming Glacier system with ice front positions from 1947 to 2016. Author provided

We used historic aerial photographs from 1966 to create an elevation map of the Fleming Glacier, and compared it to elevation measurements from 2002 to 2015. Between 1966 and 2015 the Fleming Glacier thinned by at least 100m near the front. The thinning rate, which is the elevation change rate, rapidly increased: the thinning rate after 2008 is more than twice that during 2002 to 2008, and four times the average rates from 1966 to 2008.

Ice thinning rate of the Fleming Glacier region during (a) 2002-2008 and (b) 2008-2015. Author provided

Ice flow speeds have also increased by more than 400m per year at the front since 2008. This is the largest speed change in recent years of any glacier in Antarctica. These changes all point to ice shelf collapse as the cause.

We estimate the total glacier ice volume lost from all glaciers that feed the Wordie is 179 cubic kilometres since 1966, or 319 times the volume of Sydney Harbour. The weight of this ice moving off the land and into the ocean has caused the bedrock beneath the glaciers to lift by more than 50mm.

Other research has suggested this lift could have acted to slow the glacier’s retreat, but it’s clear that the bedrock deformation has not stopped the ice movement speeding up. It seems the Fleming Glacier has a long way to go before it will return to a new stable state (in which snowfall feeding the glacier equals the ice flowing into the oceans).

Fifty years after the Wordie Ice Shelf began to collapse, the major feeding glaciers continue to thin and flow faster than before.

We can’t yet predict the full consequences of the new iceberg calving from Larsen C. But if the ice shelf does begin to retreat or collapse, history tells us it is very possible that its glaciers will flow faster – making yet more sea level rise inevitable.

Chen Zhao is a PhD student from the School of Land and Food, University of Tasmania. She receives funding from the Australian Government Research Training Program.

Christopher Watson receives funding from the Australian Research Council and the Department of Environment.

Matt King receives funding from the Australian Research Council and the Department of Environment.

The case was brought over cutting levels of nitrogen dioxide.

Chinese president Xi Jinping officially presents two giant pandas to Berlin’s zoo on Wednesday. German chancellor Angela Merkel said the bamboo-munching newcomers would be special ambassadors for the two countries. The pandas, Meng Meng and Jiao Qing, landed in Berlin on June 24 and have been settling in at the zoo since then, out of public view.

• Panda fever grips Berlin on Chinese president’s trip to Germany
  

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Mike Ashley’s ‘worth’ | American signal crayfish | String theory | Bacon sandwiches | Undergarments | Corbyn on violin

The word “earn” has become meaningless in today’s society, the word “get” being far more appropriate. In the same spirit, please could I urge you to refrain from repeating the misleading use of “worth” when referring to individuals and their personal wealth (Front page, 4 July). Mike Ashley is apparently “worth” £2.2bn – not to me he’s not.
Deirdre Burrell
Mortimer, Berkshire

• Carey Davies’s Country Diary (3 July) about the American signal crayfish in our rivers was interesting, but omitted to include one way of reducing their population: eating them. Fortunately George Monbiot has already provided information on how to do this (Monbiot cooks up revenge on invasive signal crayfish, 30 September 2009). Just make sure it’s not our (now very rare) native species.
Copland Smith
Manchester

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