Tipping elements in the Earth's climate system

Feb 04, 2008
Earth

Anthropogenic forcing could push the Earth’s climate system past critical thresholds, so that important components may “tip” into qualitatively different modes of operation. In the renowned magazine Proceedings of the National Academy of Sciences (PNAS) an international team of researchers describes, where small changes can have large long-term consequences on human and ecological systems.

“Society may be lulled into a false sense of security by smooth projections of global change,“ the researchers around Timothy Lenton from the British University of East Anglia in Norwich and Hans Joachim Schellnhuber from the Potsdam Institute for Climate Impact Research report. Global change may appear to be a slow and gradual process on human scales. However, in some regions anthropogenic forcing on the climate system could kick start abrupt and potentially irreversible changes. For these sub-systems of the Earth system the researchers introduce the term “tipping element”.

Drawing on a workshop of 36 leading climate scientists in October 2005 at the British Embassy, Berlin, Germany, a further elicitation of 52 experts in the field, and a review of the pertinent literature, the authors compiled a short-list of nine potential tipping elements. These tipping elements are ranked as the most policy-relevant and require consideration in international climate politics.

Arctic sea-ice and the Greenland Ice Sheet are regarded as the most sensitive tipping elements with the smallest uncertainty. Scientists expect ice cover to dwindle due to global warming. The West Antarctic Ice Sheet is probably less sensitive as a tipping element, but projections of its future behavior have large uncertainty. This also applies to the Amazon rainforest and Boreal forests, the El Niño phenomenon, and the West African monsoon. “These tipping elements are candidates for surprising society by exhibiting a nearby tipping point,” the authors state in the article that is published in PNAS Online Early Edition. The archetypal example of a tipping element, the Atlantic thermohaline circulation, could undergo a large abrupt transition with up to ten percent probability within this century, according to the UN climate report from 2007.

Given the scale of potentially dramatic impacts from tipping elements the researchers anticipate stronger mitigation. Concepts for adaptation that go beyond current incremental approaches are also necessary. In addition, “a rigorous study of potential tipping elements in human socio-economic systems would also be welcome,” the researchers write. Some models suggest there are tipping points to be passed for the transition to a low carbon society.

Highly sensitive tipping elements, smallest uncertainty

Greenland Ice Sheet - Warming over the ice sheet accelerates ice loss from outlet glaciers and lowers ice altitude at the periphery, which further increases surface temperature and ablation. The exact tipping point for disintegration of the ice sheet is unknown, since current models cannot capture the observed dynamic deglaciation processes accurately. But in a worst case scenario local warming of more than three degrees Celsius could cause the ice sheet to disappear within 300 years. This would result in a rise of sea level of up to seven meters.

Arctic sea-ice - As sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation than white sea-ice so that the warming is amplified. This causes more rapid melting in summer and decreases ice formation in winter. Over the last 16 years ice cover during summer declined markedly. The critical threshold global mean warming may be between 0.5 to 2 degrees Celsius, but could already have been passed. One model shows a nonlinear transition to a potential new stable state with no arctic sea-ice during summer within a few decades.

Intermediately sensitive tipping elements, large uncertainty

West Antarctic Ice Sheet - Recent gravity measurements suggest that the ice sheet is losing mass. Since most of the ice sheet is grounded below sea level the intrusion of ocean water could destabilize it. The tipping point could be reached with a local warming of five to eight degrees Celsius in summer. A worst case scenario shows the ice sheet could collapse within 300 years, possibly raising sea level by as much as five meters.

Boreal forest - The northern forests exhibit a complex interplay between tree physiology, permafrost and fire. A global mean warming of three to five degrees Celsius could lead to large-scale dieback of the boreal forests within 50 years. Under climate change the trees would be exposed to increasing water stress and peak summer heat and would be more vulnerable to diseases. Temperate tree species will remain excluded due to frost damage in still very cold winters.

Amazon rainforest - Global warming and deforestation will probably reduce rainfall in the region by up to 30 percent. Lengthening of the dry season, and increases in summer temperatures would make it difficult for the forest to re-establish. Models project dieback of the Amazon rainforest to occur under three to four degrees Celsius global warming within fifty years. Even land-use change alone could potentially bring forest cover to a critical threshold.

El Niño Southern Oscillation (ENSO) – The variability of this ocean-atmosphere mode is controlled by the layering of water of different temperatures in the Pacific Ocean and the temperature gradient across the equator. During the globally three degrees Celsius warmer early Pliocene ENSO may have been suppressed in favor of persistent El Niño or La Niña conditions. In response to a warmer stabilized climate, the most realistic models simulate increased El Niño amplitude with no clear change in frequency.

Sahara/Sahel- and West African monsoon - The amount of rainfall is closely related to vegetation climate feedback and sea surface temperatures of the Atlantic Ocean. Greenhouse gas forcing is expected to increase Sahel rainfall. But a global mean warming of three to five degrees Celsius could cause a collapse of the West African monsoon. This could lead either to drying of the Sahel or to wetting due to increased inflow from the West. A third scenario shows a possible doubling of anomalously dry years by the end of the century.

Indian summer monsoon - The monsoon circulation is driven by a land-to-ocean pressure gradient. Greenhouse warming tends to strengthen the monsoon since warmer air can carry more water. Air pollution and land-use that increases the reflection of sunlight tend to weaken it. The Indian summer monsoon could become erratic and in the worst case start to chaotically change between an active and a weak phase within a few years.

Lowly sensitive tipping elements, intermediate uncertainty

Atlantic thermohaline circulation - The circulation of sea currents in the Atlantic Ocean is driven by seawater that flows to the North Atlantic, cools and sinks at high latitudes. If the inflow of freshwater increases, e.g. from rivers or melting glaciers, or the seawater is warmed, its density would decrease. A global mean warming of three to five degrees Celsius could push the element past the tipping point so that deep water formation stops. Under these conditions the North Atlantic current would be disrupted, sea level in the North Atlantic region would rise and the tropical rain belt would be shifted.

Citation: Article: Lenton, T. M., Held, H., Kriegler, E., Hall, J. W., Lucht, W., Rahmstorf, S. and Schellnhuber, H. J. (2008). Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences, Online Early Edition

Source: Potsdam Institute for Climate Impact Research

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fleem
3 / 5 (4) Feb 04, 2008
Pretty impressive how man is capable of pushing the climate with far more force than any volcanism, solar variations, internal magma flow variations, and meteorite strikes that have occurred over the last 500 million years. Next we'll be told that cell phone use is sending the Earth careening into the sun.
pauldentler
3 / 5 (4) Feb 04, 2008
Fleem, the writer forgot to mention what they are finding at the bottom of the melted ice, they have found "very old vegetation", some of it that had grown there over 2,000 & more years ago. Yeah, the writer wanted us to think the current ice melt is "manmade", so he purposely left out the factoid about the vegetation that was growing there before it was smothered by ice that had previously not existed.
barakn
3.7 / 5 (3) Feb 05, 2008
Pauldentler, you're probably referring to the Greenland ice core, but you got your numbers wrong. By about 1,998,000 years. http://usinfo.sta...265.html
Soylent
3.7 / 5 (3) Feb 05, 2008
"Fleem, the writer forgot to mention what they are finding at the bottom of the melted ice, they have found "very old vegetation", some of it that had grown there over 2,000 & more years ago."

Strawman. There's nothing especially good or unique about our climate that requires preserving and no one is arguing this. What is argued is that the pace of change is too high and will cause the collapse of many existing eco systems.

"Pretty impressive how man is capable of pushing the climate with far more force than any volcanism, solar variations, internal magma flow variations, and meteorite strikes that have occurred over the last 500 million years."

Fleem, argument from personal incredulity caries no weight unless you back it up with something.

Volcanism releases a tiny fraction, on average less than 1%, of current CO2-emissions. The particulates from major erruptions produce a tiny but abrupt change in global temperatures; it's within tolerances of eco systems without requiring adaption. Super-volcanoes have been implicated in mass-extinctions but no erruptions have occured within historic times.

Solar variations are known from the tree ring records and direct observations in recent times and have been incapable of explaining the current period of warming. Solar variations over periods of millions of years have been huge, but they have been gradual, allowing eco systems to adapt.

Internal magma flow variations have time scales of tens of thousands of years and are of little consequence for climate other than through volcanism.

Meteorite strikes are much too small to cause any kind of climate change. Assuming you mean asteroids, yes they've caused mass extinctions and abrupt change of climate. Remind me again who is arguing anthropogenic climate change is worse than a major asteroid impact like the one that got the dinosaurs?
fleem
1 / 5 (1) Feb 05, 2008
"Remind me again who is arguing anthropogenic climate change is worse than a major asteroid impact like the one that got the dinosaurs?"

If there are undesirable metastable states in the Earth's climate, as this article suggests, why haven't any of the catastrophes that have occurred over the last 500 million years placed the Earth in one of those undesirable metastable states?

"Volcanism releases a tiny fraction, on average less than 1%, of current CO2-emissions."

I said nothing of the kind--unless your definition of "current volcanism" includes the last 500 million years. In which case its just a tad over 1% (try 98%).

"Solar variations are known from the tree ring records and direct observations in recent times and have been incapable of explaining the current period of warming."

You are incorrect. First, tree ring records go back no more than a couple hundred years (in a few cases several hundred)--so they can only tell us about very recent variations like the tail end of the little ice age--which are simply short term pseudo random variations in the climate caused by many factors and which are orders of magnitude more pronounced than the data we want. The signal is orders of magnitude below the noise. You'd need orders of magnitude older trees to integrate solar variations out of all that short-term noise. Second, our satellite solar sensors (which are several orders of magnitude more precise than tree rings) DO show EXACTLY the solar output increase that accounts for a .7C increase (extrapolated) over the last century. Granted, the necessary precision is barely at the noise floor even in those sensors and I admit the jury is still out, but its the only data we have that is anywhere near the accuracy we need. I can show you detailed numbers on this.

"Internal magma flow variations have time scales of tens of thousands of years and are of little consequence for climate other than through volcanism."

Incorrect. Take a look at Yellowstone, which has shown a notable variation within the last century. Such variations, some far more powerful, over the last 500 million years could easily push the climate harder than man ever could. Granted, venting--especially cataclysmic--even overshadows that.

Soylent you've made it clear you know effectively nothing about geophysical history. I suggest you quit while you're behind.

pauldentler
5 / 5 (1) Feb 05, 2008
Pauldentler, you're probably referring to the Greenland ice core, but you got your numbers wrong. By about 1,998,000 years. [url][url]http://usinfo.sta...265.html[/url][/url]
Pauldentler, you're probably referring to the Greenland ice core, but you got your numbers wrong. By about 1,998,000 years. [url][url]http://usinfo.sta...265.html[/url][/url]
Pauldentler, you're probably referring to the Greenland ice core, but you got your numbers wrong. By about 1,998,000 years. [url][url]http://usinfo.sta...265.html[/url][/url]
pauldentler
5 / 5 (1) Feb 05, 2008
Nope, I'm referring to the radio carbon dating on the vegetation that was found at the bottom of the Baffin Island ice melt. You can't radio carbon date as far back as 1,998,000 years (I know my nuclear physics, would you like to know how many semesters of calculus I've had?).

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