What do climate scientists say that recently obtained data about our past climate tells us about the consequences of long-term increases in atmospheric CO2 (and other GHGs)? They tell us that we are already at levels that, if sustained (and at current sink rates it seems that CO2 has an atmospheric half-life of fifty years or so), the result will be an Earth without ice caps if we are about 350 ppm - which we already are, and heading north rapidly.
At the December 2007 meeting of the American Geophysical Union (AGU), according to the Washington Post NASA's Jim Hansen "offered a simple, straightforward and mind-blowing bottom line for the planet: 350, as in parts per million carbon dioxide in the atmosphere". http://www.washingtonpost.com/wp-dyn/content/article/2007/12/27/AR2007122701942.html. The WaPo report provided further background:
Twenty years ago, Hansen kicked off this issue by testifying before Congress that the planet was warming and that people were the cause. At the time, we could only guess how much warming it would take to put us in real danger. Since the pre-Industrial Revolution concentration of carbon in the atmosphere was roughly 275 parts per million, scientists and policymakers focused on what would happen if that number doubled -- 550 was a crude and mythical red line, but politicians and economists set about trying to see if we could stop short of that point. The answer was: not easily, but it could be done.
In the past five years, though, scientists began to worry that the planet was reacting more quickly than they had expected to the relatively small temperature increases we've already seen. The rapid melt of most glacial systems, for instance, convinced many that 450 parts per million was a more prudent target. That's what the European Union and many of the big environmental groups have been proposing in recent years, and the economic modeling makes clear that achieving it is still possible, though the chances diminish with every new coal-fired power plant.
But the data just keep getting worse. The news this fall that Arctic sea ice was melting at an off-the-charts pace and data from Greenland suggesting that its giant ice sheet was starting to slide into the ocean make even 450 look too high. Consider: We're already at 383 parts per million, and it's knocking the planet off kilter in substantial ways. So, what does that mean?
It means, Hansen says, that we've gone too far. "The evidence indicates we've aimed too high -- that the safe upper limit for atmospheric CO2 is no more than 350 ppm," he said after his presentation. Hansen has reams of paleo-climatic data to support his statements (as do other scientists who presented papers at the American Geophysical Union conference in San Francisco this month). The last time the Earth warmed two or three degrees Celsius -- which is what 450 parts per million implies -- sea levels rose by tens of meters, something that would shake the foundations of the human enterprise should it happen again.
Hansen later released a paper online that discusses his views (including his policy suggestions, which I reserve for later - but readers should feel free to take a peek). The paper is posted at Hansen's Columbia University webpage (along with others of possible interest): http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf
What does Hansen conclude? Here are some key excerpts:
- The approximate equilibrium characterizing most of Earth’s history is unlike the current situation, in which GHGs are rising at a rate much faster than the coupled climate system can respond.
- Paleoclimate data show that long-term climate has high sensitivity to climate forcings and that the present global mean CO2, 385 ppm, is already in the dangerous zone (including substantial effects that are "built-in" but yet to be felt).
- Paleoclimate data show that climate sensitivity is ~3°C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO2 for the range of climate states between glacial conditions and icefree Antarctica.
- No additional forcing is required to raise global temperature to at least the level of the Pliocene, 2-3 million years ago, a degree of warming that would surely yield ‘dangerous’ climate impacts.
- If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.
- Stabilizing atmospheric CO2 and climate requires that net CO2 emissions approach zero, because of the long lifetime of CO2.
Further background discussion includes the following:
- Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less.
- Rapid on-going climate changes and realization that Earth is out of energy balance, imply that more warming is ‘in the pipeline’, add urgency to investigation of the dangerous level of GHGs.
- GHG and surface albedo changes are mechanisms causing the large global climate changes in Fig. 1, but they do not initiate these large climate swings. Instead changes of GHGs and sea level (a measure of ice sheet size) lag temperature change by typically several hundred years. GHG and surface albedo changes are positive climate feedbacks. Major glacial-interglacial climate swings are instigated by slow changes of Earth’s orbit, especially the tilt of Earth’s spinaxis relative to the orbital plane and the precession of the equinoxes that influences the intensity of summer insolation. Global radiative forcing due to orbital changes is small, but ice sheet size is affected by changes of geographical and seasonal insolation [e.g., ice melts at both poles when the spin-axis tilt increases, and ice melts at one pole when perihelion, the closest approach to the sun, occurs in late spring]. Also a warming climate causes net release of
GHGs. The most effective GHG feedback is release of CO2 by the ocean, due partly to temperature dependence of CO2 solubility but mostly to increased ocean mixing in a warmer climate, which acts to flush out deep ocean CO2 and alters ocean biological productivity. GHG and surface albedo feedbacks respond and contribute to temperature change caused by any climate forcing, natural or human-made, given sufficient time.
- Paleoclimate data permit evaluation of long-term sensitivity to specified GHG change. Plotting GHG forcing from ice core data against temperature shows that global climate sensitivity including the slow surface albedo feedback is 1.5°C per W/m2 or 6°C for doubled CO2 (Fig. 2), twice as large as the Charney fast-feedback sensitivity. This long-term climate sensitivity is relevant to GHGs that remain airborne for centuries-tomillennia. GHG amounts will decline if emissions decrease enough, but, on the other hand, if the globe warms much further, carbon cycle models and empirical data find a positive GHG feedback. Amplification of GHGs is moderate if warming is kept within the range of recent interglacial periods, but larger warming risks greater release of CH4 and CO2 from methane
hydrates in tundra and ocean sediments.
- Human-made global climate forcings now prevail over natural forcings. Earth may have entered the Anthropocene era 6-8 ky ago, but the net human-made forcing was small, perhaps slightly negative, prior to the industrial era. GHG forcing overwhelmed
natural and negative human-made forcings only in the past quarter century.
- How long does it take to reach equilibrium temperature? Response is slowed by ocean thermal inertia and the time needed for ice sheets to disintegrate.
- The expanded time scale for the industrial era (Fig. 2) reveals a growing gap between actual global temperature (purple curve) and equilibrium (long-term) temperature response based on the net estimated forcing (black curve). Ocean and ice sheet response times together account for this gap, which is now 2.0°C. Climate models, which include only fast feedbacks, have additional warming of
~0.6°C in the pipeline today because of ocean thermal inertia. The remaining gap between equilibrium temperature for current atmospheric composition and actual global temperature is ~1.4°C. This further 1.4°C warming to come is due to the slow surface albedo feedback, specifically ice sheet disintegration and vegetation change.
- Present-day observations of Greenland and Antarctica show increasing surface melt, loss of buttressing ice shelves, accelerating ice streams, and increasing overall mass loss. These rapid changes do not occur in existing ice sheet models, which are missing critical
physics of ice sheet disintegration. Sea level changes of several meters per century occur in the paleoclimate record, in response to forcings slower and weaker than the present human-made forcing. It seems likely that large ice sheet response will occur within centuries, if human-made forcings continue to increase. Once ice sheet disintegration is underway, decadal changes of sea level may be substantial.
- GHGs other than CO2 cause climate forcing comparable to that of CO2, but growth of non-CO2 GHGs is falling below IPCC scenarios and the GHG climate forcing change is determined mainly by CO2. Net human-made forcing is comparable to the CO2
forcing, as non-CO2 GHGs tend to offset negative ice-free aerosol forcing.
- Theory and models indicate that subtropical regions expand poleward with global warming. Data reveal a 4-degree latitudinal shift already, larger than model predictions, yielding increased aridity in southern United States, the Mediterranean region, Australia and parts of Africa. Impacts of this climate shift support the conclusion that 385 ppm CO2 is already deleterious.
- Alpine glaciers are in near-global retreat. After a flush of fresh water, glacier loss foretells long summers of frequently dry rivers, including rivers originating in the Himalayas, Andes and Rocky Mountains that now supply water to hundreds of millions of people. Present glacier retreat, and warming in the pipeline, indicate that 385 ppm CO2 is already a threat.
- Equilibrium sea level rise for today’s 385 ppm CO2 is at least several meters, judging from paleoclimate history. Accelerating mass losses from Greenland and West Antarctica heighten concerns about ice sheet stability.
- Stabilization of Arctic sea ice cover requires restoration of planetary energy balance. Climate models driven by known forcings yield a present planetary energy imbalance of +0.5-1 W/m2, a result supported by observed increasing ocean heat content. CO2 amount must be reduced to 325-355 ppm to increase outgoing flux 0.5-1 W/m2, if other forcings are unchanged. A further reduced flux, by ~0.5 W/m2, and thus CO2 ~300-325 ppm, may be needed to restore sea ice to its area of 25 years ago.
- Coral reefs are suffering from multiple stresses, with ocean acidification and ocean warming principal among them. Given additional warming ‘in-the-pipeline’, 385 ppm CO2 is already deleterious.