A Very Inconvenient Truth(超訳:超不都合な真実)

Here, we discuss reasons why the IPCC process is prone to underestimating the threats of global climate change.


the European Union decided to adopt an average global temperature increase of 2.0°C as its threshold for dangerous climate change (European Commission, 2005).
Despite the uncertainties, this threshold was also agreed to at the December 2009 United Nations Climate Change Conference of the Parties 15 (COP 15) in Copenhagen, Denmark.

As policymakers continue to discuss emission scenarios and target values for the GHG stabilization level, the concept of warming in the pipeline (Hansen et al., 2005) has taken on increasing importance. Warming in the pipeline corresponds to the growing gap between the observed value of average global temperature and the expected equilibrium value once various feedback mechanisms are taken into account.

At present, average global temperature has increased by ~ 0.8°C above late nineteenth century values. Ramannthan and Feng (2008) suggest that this increase is roughly consistent with 25% of the committed warming in the pipeline that IPCC models project will unfold during the twenty-first century as heat currently stored in the upper ocean approaches equilibrium with the atmosphere and as the cooling effects of aerosols in the atmosphere are reduced by air pollution abatement laws.

Thus, they conclude that, even if GHG emissions were to drop precipitously and concentrations were to stabilize at today’s levels, we are already committed to a temperature increase of 2.4°C by the end of the century, a warming in excess of the European Union’s threshold for dangerous climate change.
Of course, GHG emissions will not drop precipitously tomorrow and in fact have been increasing more rapidly during recent years than the worstcase scenarios used in previous IPCC reports (Raupach et al., 2007).

Because there are physical limits on the rate at which new, low-carbon energy technologies can be deployed (Kramer and Haigh, 2009), even assuming that the proper financial incentives are adopted (Galiana and Green, 2009), emissions are unlikely to be reduced substantially for several decades.

Therefore, we can anticipate that GHG concentrations will continue to rise for at least the first half of the twenty-first century before eventually stabilizing. Because of CO2’s long residence time in the atmosphere, the overall GHG concentration in the absence of anthropogenic sequestration efforts will stabilize for the next thousand years at a level that is approximately 40% of its peak enhancement over the pre-industrial period (Solomon et al.,2009). Even more significantly, Solomon et al. (2009) conclude that the climate warming induced by elevated GHG concentrations is largely irreversible.

Once atmospheric temperature reaches equilibrium at a certain peak-overall GHG concentration, it will not drop markedly for the next thousand years even as GHG concentrations decline. This irreversibility comes about because the atmosphere’s loss of heat to the ocean is even more gradual than its loss of CO2.
The thermal inertia of the ocean, which is delaying the rate of climate warming today, will delay the rate of climate cooling in the future. A crucial point for policymakers and the public to recognize is that the global GHG stabilization level reached during the twenty-first century will have climatic consequences for the remainder of the millennium.

The short- and long-term consequences of a particular GHG stabilization level are uncertain due to our limited understanding of the slower feedback mechanisms in the climate system. For example, the committed warming in the pipeline for the twenty-first century discussed previously includes only the relatively rapid feedback mechanisms associated with the ocean’s thermal inertia and atmospheric aerosols; it does not take into account the more gradual surface-albedo feedback mechanisms associated with disintegration of the cryosphere and changes in vegetation cover (Hansen et al., 2008).
Using estimates from the paleorecord, Hansen et al. (2008) argue that the net GHG radiative forcing, including those latter feedback mechanisms, can be twice as high when the cryosphere is in a state comparable to that of today. From these arguments, as well as recent observations of rapid ice sheet disintegration in Greenland (Chen et al., 2006; Howat et al., 2007) and West Antarctica (Rignot and Jacobs, 2002), Hansen et al. (2008) conclude that sea-level rise during the next century might greatly exceed estimates provided by the IPCC Fourth Assessment Report (FAR; IPCC, 2007a,b,c). Lending further support to this conclusion is a recent study reporting that Yucatán coral reefs experienced a sea-level rise of ~ 3 m in a century during the previous interglacial period, approximately 121,000 years ago (Blanchon et al., 2009).

A sea-level rise during the twenty-first century comparable to that reported for the last interglacial period would be catastrophic to human society. However, the Greenland and West Antarctic ice sheets are only two among a number of tipping elements that are considered at risk from climate warming during the next few centuries (Lenton et al., 2007). A tipping element is a large-scale component of the Earth system that has the potential to rapidly change state in response to small perturbations that exceed some critical threshold—its tipping point. Society is especially vulnerable to these types of nonlinear responses to climate warming because their speed and magnitude make adaptation difficult, if not impossible. Among the policy-relevant tipping elements Lenton et al. (2007) evaluated for their sensitivity to future climate warming, those associated with melting of the cryosphere posed the most imminent threats. Many climate scientists now anticipate a complete seasonal disappearance of Arctic sea ice in the coming decades (Serreze et al., 2007; Wang and Overland, 2009) and significant losses from the Greenland and West Antarctic ice sheets during the next two centuries (Hansen et al., 2005, 2008).
Although there are many precedents in the paleorecord of the global meridional overturning circulation (MOC) shutting down in response to major cryospheric disintegration events (Greene et al., 2008), its potential occurrence during the twenty-first century is considered unlikely (Weaver and Hillaire-Marcel, 2004). However, a slowing down of the MOC is considered likely during the present century, and a continued rise in GHG forcing will increase the risk of a complete shutdown in the coming centuries.

For policymakers, there are two important messages to take home from the recent scientific findings that have emerged since the IPCC FAR.
First, the climate system is less resilient to GHG forcing than we previously thought (Alison et al., 2009; Sokolov et al., 2009). Greater climate sensitivity to GHG forcing makes the system less resistant to warming, while the ocean’s thermal inertia makes that warming essentially irreversible for the next thousand years.
Second, there is convincing evidence that the committed warming in the pipeline will not only exceed the European Union’s 2.0°C threshold for dangerous climate change by the end of the century, it may have already exceeded the tipping point for destabilizing Earth’s cryosphere.

Clickで救えるblogがある⇒人気blogランキングにほんブログ村 環境ブログ 環境学へ