Third in a series
Danger from implied temperature increase
The current level of atmospheric CO2 only is sufficient to increase the global temperature at equilibrium by +1.5°C, based on the standard assumption of near-term climate sensitivity of 3°C for doubled CO2.
If all current greenhouse gases are taken into account, then:
The observed increase in the concentration of greenhouse gases (GHGs) since the pre-industrial era has most likely committed the world to a warming of 2.4°C (within a range of +1.4°C to +4.3°C) above the pre-industrial surface temperatures. (Ramanthan and Feng)And the 2007 IPCC Synthesis report (Table 5.1 on emission scenarios) also shows that for levels of greenhouse gases that have already been achieved (CO2 in the range of 350–400 ppm, CO2e in the range 445–490 ppm) and peaking by 2015, the likely temperature rise is in the range of 2–2.4°C.
These scenarios include short-lived gases such as methane, which degrades out of the atmosphere in a decade, and also nitrous oxide, which has an atmospheric lifetime of around a century. On the other hand, the fact that temperatures are not already much higher than they are today is due principally to the large-scale emission of very short-lived (10 days) aerosols such as soot and exhausts from burning fossil fuels, industrial pollution and dust storms, which are providing temporary cooling. The effect is known popularly as “global dimming”, because the overall aerosol impact is to reduce, or dim, the sun’s radiation, thus masking some of the heating effect of greenhouse gases. The aerosol impact is not precisely known, but Ramanthan and Feng estimate it as high as ~1°C. As the world moves to low-emission technologies, most of the aerosols and their temporary cooling will be lost. Recent research finds that quickly eliminating all greenhouse gas emissions (and necessarily the associated aerosols) would produce warming of between 0.25 and 0.5 °C over the decade immediately following (Matthews and Zickfield; Hansen, Sato et al.).
A practical consideration of “dangerous” can include the question as to whether there are tipping points or “concerns” activated for the elevated temperatures that we are generally considered to be already committed to: conservatively in the range say +1.5 to 2°C and, more pragmatically, in the range of 2 to 2.4°C if all current greenhouse gases are considered. A related question is whether the +1.5°C goal advocated by the small island states and surveyed recently by Climate Action Network Europe and Climate Analytics would avoid “dangerous” climate change and significant tipping points.
This is a broad topic, but four recent important research findings on impacts for the current committed warming are arresting:
Greenland Ice Sheet tipping point
The tipping point for GIS has been revised down by Robinson, Calov et al. to +1.6ºC (uncertainty range of +0.8-+3.2ºC) above pre-industrial, just as regional temperatures are increasing at three-to-four times faster than the global average, and the increased heat trapped in the Arctic due to the loss of reflective sea ice ensures an acceleration in the Greenland melt rate. If the lower Greenland boundary in the uncertainty range turned out to be right, then with current warming of +0.8ºC over pre-industrial we have already reached Greenland’s tipping point. And, with temperature rises in the pipeline, the upward trajectory of annual greenhouse gas emissions, the projected future increases in fossil fuel use, and the continuing political impasse in international climate negotiations, we are very likely to hit the best estimate of +1.6ºC within a decade or two at most.
Coral reefs
Frieler, Meinshausen et al. show that “preserving more than 10 per cent of coral reefs worldwide would require limiting warming to below +1.5°C (atmosphere–ocean general circulation models (AOGCMs) range: 1.3–1.8°C) relative to pre-industrial levels”. Obviously at less than 10 per cent, the reefs would be remnant, and reef systems as we know them today would be a historical footnote. Already, the data suggests that the global area of reef systems has already been reduced by half. A sober discussion of coral reef prospects can be found in Roger Bradbury’s “A World Without Coral Reefs” and Gary Pearce’s “Zombie reefs as a harbinger for catastrophic future”. The opening of Bradbury’s article is to the point:
It’s past time to tell the truth about the state of the world’s coral reefs, the nurseries of tropical coastal fish stocks. They have become zombie ecosystems, neither dead nor truly alive in any functional sense, and on a trajectory to collapse within a human generation. There will be remnants here and there, but the global coral reef ecosystem — with its storehouse of biodiversity and fisheries supporting millions of the world’s poor — will cease to be.3c. Arctic carbon stores
As Climate Progress recently noted: “We’ve known for a while that ‘permafrost’ was a misnomer” because thawing permafrost feedback will turn the Arctic from a net carbon sink to a net source in the 2020s and defrosting permafrost will likely add up to 1ºC to total global warming by 2100. A 2012 UNEP report on Policy implications of warming permafrost says the recent observations “indicate that large-scale thawing of permafrost may have already started.” In February 2013, scientists using radiometric dating techniques on Russian cave formations to measure historic melting rates warned that a +1.5ºC global rise in temperature compared to pre-industrial was enough to start a general permafrost melt. Vaks, Gutareva et al. found that “global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.” Vaks says that: “1.5ºC appears to be something of a tipping point”.
Previously a study of East Siberian permafrost by Khvorostyanov, Ciais et al. found that once mobilised, the process would be self-maintaining due to “deep respiration and methanogenesis” (formation of methane by microbes). In other words, the microbial action that produces methane as the carbon stores melt would produce sufficient heat to maintain the process: “once active layer deepening in response to atmospheric warming is enough to trigger deep-soil respiration, and soil microorganisms are activated to produce enough heat, the mobilization of soil carbon can be very strong and self-sustainable”.
A sharp scientific debate has started on the stability of large methane clathrate stores just below the ocean floor on the shallow East Siberian Sea, following the publication in July 2013 of research by Whiteman, Hope and Wadhams which said that the release of a single giant “pulse” of methane from thawing Arctic permafrost beneath the East Siberian sea could come with a $60 trillion global price tag. Wadhams says “the loss of sea ice leads to seabed warming, which leads to offshore permafrost melt , which leads to methane release, which leads to enhanced warming, which leads to even more rapid uncovering of seabed”, and this is not “a low probability event”.
Multiple targets reduce allowable warming
Steinacher, Joos et al. explore the interaction of targets in emissions reductions, focussing on the 2ºC temperature goal. They find that when multiple climate targets are set (such as food production capacity, ocean acidity, atmospheric temperature), “allowable cumulative emissions are greatly reduced from those inferred from the temperature target alone”. In fact, “When we consider all targets jointly, CO2 emissions have to be cut twice as much as if we only want to meet the 2ºC target.”
Lessons from climate history
Another fruitful line of inquiry on whether climate change is already “dangerous” is to look at the paleo-climate (climate history) record for circumstances analogous to present conditions to learn what planetary and climate conditions were like at that time. With current CO2 levels at 400 ppm, a useful comparison is the Pliocene (3–5 million years ago). The research body is large and growing in this area, but here are some examples:
Sea-levels
Rohling, Grant et al. find that during the mid-Pliocene, when greenhouse gases were similar to today, sea levels were more than 20 metres higher than today “we estimate sea level for the Middle Pliocene epoch (3.0–3.5 Myr ago) – a period with near-modern CO2 levels – at 25±5 metres above present, which is validated by independent sea-level data”. Likewise Hansen, Sato et al. find that “during the middle-Pliocene… we find sea level fluctuations of 20-40 metres associated with global temperature variations between today’s temperature and +3°C”.
Speed of sea-level rise
The speed of sea-level rise may far exceed the current, rather reticent estimates that are used for policy purposes. Blancon, Eisenhauer et al. examined the paleo-climate record and showed a sea-level rises of 3 metres in 50 years due to the rapid melting of ice sheets 123,000 years ago in the Eemian, when the energy imbalance in the climate system was less than at present.
Polar feedbacks
Hansen, Sato et al. find that current temperatures are at least as high as the Holocene Maximum (i.e., as high as they have been over the last 10,000 years). They sum up:
Earth at peak Holocene temperature is poised such that additional warming instigates large amplifying high-latitude feedbacks. Mechanisms on the verge of being instigated include loss of Arctic sea ice, shrinkage of the Greenland ice sheet, loss of Antarctic ice shelves, and shrinkage of the Antarctic ice sheets. These are not runaway feedbacks, but together they strongly amplify the impacts in polar regions of a positive (warming) climate forcing… Augmentation of peak Holocene temperature by even +1ºC would be sufficient to trigger powerful amplifying polar feedbacks, leading to a planet at least as warm as in the Eemian and Holsteinian periods, making ice sheet disintegration and large sea level rise inevitable.[It is relevant here to note that warming in the pipeline due to thermal inertia, plus warming associated with the loss of aerosols, is greater than +1ºC.]
And during the Pliocene, with atmospheric greenhouse levels similar to today, the northern hemisphere was free of glaciers and ice sheets and beech trees grew in the Transantarctic Mountains. There are also strong indications that permanent El Nino conditions prevailed.
4d. Arctic carbon stores
As discussed above, scientists using radiometric dating techniques on Russian cave formations to measure historic melting rates going back 500,000 years conclude that a +1.5ºC global rise in temperature compared to pre-industrial is enough to initiate widespread permafrost melt.
In May this year, Brigham-Grette, Melles et al. published evidence from Lake El’gygytgyn, in north-east Arctic Russia, showing that 3.6–3.4 million years ago, summer mid-Pliocene temperatures locally were ~8°C warmer than today, when CO2 was ~400 ppm. This is highly significant because researchers including Celia Bitz and Philippe Ciais have previously found that the tipping point for the large-scale loss of permafrost carbon is around +8ºC to 10ºC regional temperature increase. Caias told the March 2009 Copenhagen climate science conference that: “A global average increase in air temperatures of +2ºC and a few unusually hot years could see permafrost soil temperatures reach the +8ºC threshold for releasing billions of tonnes of carbon dioxide and methane”. So, if the current level of greenhouse gases is enough to produce Arctic regional warming of ~+8°C and that is a likely tipping point for large-scale permafrost loss, we have reached a disturbing milestone.
Even more disturbing is new research from Ballantyne, Axford et al. which says that during the Pliocene epoch, when CO2 levels were ~400 ppm, Arctic surface temperatures were 15-20°C warmer than today’s surface temperatures. They suggest that much of the surface warming likely was due to ice-free conditions in the Arctic. Compared to the estimated tipping point for the large-scale loss of permafrost carbon of +8º– 10ºC regional warming, this research confirms both that the current level of greenhouse gases is sufficient to both create a sea-ice free Arctic, and Arctic warming more than sufficient to trigger large-scale loss of permafrost carbon.
Next post: Climate safety and the emissions reduction challenge
