What constitutes “dangerous” climate change?

by Judith Curry

The United Nations Framework Convention on Climate Change (UNFCCC ) international environmental treaty (1992) states as its objective:

The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.

As stated by the Climate Change Emergency Medical Response (an organization that is trying to grapple with the problem of dangerous climate change):

The position of Annex 1 signatory countries (industrialized nations), the UNFCCC Secretariat, and the IPCC is that “dangerous,” in the context of interference with the climate system, is not yet defined.

In spite of the lack of definition of “dangerous” climate change, we have international and national policies on the table targeted at stabilization of greenhouse gases to prevent “dangerous” climate change.  We also see statements from scientists and others that “dangerous” climate change is already happening.

So, what actually constitutes “dangerous” climate change?  The answer depends on societal values and vulnerability/resilience, which vary regionally and culturally and even among individuals within regions and culture.  We need to understand these issues before implementing far reaching policies to avoid “dangerous” climate change, even if our understanding of future climate change was perfect.

Reasons for concern

The IPCC 3rd and 4th Assessment reports refer to “reasons for concern” (summarized by Pew Climate):

• Risk to unique and threatened systems (e.g., coral reefs, tropical mountain glaciers, endangered species, etc.)
• Risk of extreme weather events (e.g., heat waves, floods, droughts, wildfires, hurricanes)
• Disparities of impacts and vulnerabilities (e.g., disproportionate harm to developing countries and the poor in developed countries)
• Aggregate damages (i.e. net global market damages)
• Risks of large-scale discontinuities (e.g., rapid sea-level rise, ocean acidification, and strong amplifiers of warming)

Authors of the IPCC AR4 subsequently published a paper in PNAS  which included the “burning embers” diagram that revised the sensitivities of the reasons for concern to increases in global average temperature based on a more thorough understanding of the concept of vulnerability.

IPCC WGII Chapter 19 identifies  “key vulnerabilities”  as being associated with  food supply, infrastructure, health, water resources, coastal systems, ecosystems, global biogeochemical cycles, ice sheets, and modes of oceanic and atmospheric circulation. Key vulnerabilities were identified using the following criteria:

• magnitude of impacts
• timing of impacts
• persistence and reversibility of impacts
• likelihood (estimates of uncertainty) of impacts and vulnerabilities and confidence in those estimates
• potential for adaptation
• distributional aspects of impacts and vulnerabilities
• importance of the system(s) at risk.

This treatment is still rather vague and doesn’t address what actually constitutes dangerous climate change.

Other perspectives

Hansen et al. (2007) define dangerous in the context of Arctic climate change, tropical cyclone intensity, and ice sheet stability.  They conclude:

Identification of “dangerous” effects is partly subjective, but we find evidence that added global warming of more than 1C above the level in 2000 has effects that may be highly disruptive.

Schellnhuber, author of Avoiding Dangerous Climate Change, states:

‘One school of thought has been that climate change caused by human activities will be gradual and we will be able to adapt to it. However, I believe the evidence now indicates that once climate change exceeds certain “tipping points” or critical thresholds, the consequences will enter a largely uncontrollable and irreversible domain.’ He says there is a real possibility that we will see a range of major large-scale events that will be beyond our management. The dangers, says Schellnhuber, include intensification of El Niño and the risk that it could become a permanent feature, weakening of the Gulf Stream, melting of the West Antarctic and Greenland ice sheets, disruption of the Indian monsoon, widespread death of corals due to bleaching, and hurricanes of increased intensity. Without action on our part, these tipping points could occur one by one, some sooner, some later.

The Stern Review on the Economics of Climate Change provides a comprehensive analysis of the economic impacts of climate change.  Main conclusions on the economic impacts are:

• Climate change threatens the basic elements of life for people around the world — access to water, food production, health, and use of land and the environment.
• The impacts of climate change are not evenly distributed — the poorest countries and people will suffer earliest and most. And if and when the damages appear it will be too late to reverse the process. Thus we are forced to look a long way ahead.
• Climate change may initially have small positive effects for a few developed countries, but it is likely to be very damaging for the much higher temperature increases expected by mid-to-late century under BAU scenarios.

Obertsteiner et al. (2001) provides the best articulation of possible danger from climate change that I’ve come across:

“[N]on-linear response processes of natural ecosystems transmitted through complex cause-effect chains would lead to a sudden upward shift in the level of climate related damages and disasters that finally result in civil unrest in some regions of the world as those societies lost their capacity to deal with the additional climate risk(s).”

External and internal definitions of danger

Dessai et al. (2004) provide what I find to be the most lucid analysis of the issues surrounding defining dangerous climate change. External definitions of dangerous climate change are those provided by researchers:

Danger measured through threshold in physical vulnerability (top down approach):

1. Large-scale eradication of coral reef systems (O’Neil and Oppenheimer, 2002)
2. Disintegration of the West Antarctic Ice Sheet (Vaughan and Spouge, 2002)
3. Breakdown of the thermohaline circulation (Rahmstorf, 2000)
4. Qualitative modification of crucial climate-system patterns such as ENSO and NAO (Timmermann et al., 1999)
5. Climate change exceeding the rate at which biomes can migrate (Malcom and Markham, 2000)

Danger measured through threshold in social vulnerability (bottom up approach):

1. Irrigation demand exceeding 50 per cent of annual seasonal water usage for agriculture in northern Victoria, Australia (Jones, 2000)
2. Depopulation of sovereign atoll countries (Barnett and Adger, 2003)
3. Additional millions of people at risk from water shortage, malaria, hunger and coastal flooding (Parry et al., 2001)
4. Destabilisation of international order by environmental refugees and emergence of conflicts (Homer-Dixon, 1991; Barnett, 2003)
5. World impacts exceeding a threshold percentage of GDP (Fankhauser, 1995; Nordhaus and Boyer, 2000)

Internal definitions of danger  are based on psychological, social, moral, institutional and cultural processes that influence perceptions o about what constitutes danger and significant impact associated with  the perceived insecurity arising from impacts associated with changing extreme weather events, and often immediate threats to life and livelihood.

How to proceed?

In my opinion, the primary reason that the UNFCCC has been unable to define “dangerous” anthropogenic climate change is because they have framed the problem and its solution to be irreducibly global.  If the problem is viewed as an aggregate of regional problems in the context of a bottom-up incremental policy approach such as that promoted by Ron Brunner, then presumably a more meaningful understanding of dangerous climate change could emerge, which would be a source of political will for actually addressing the problems.

More importantly, extreme weather events and natural climate variability have adverse impacts, which in some instances (time-space) may counter the impacts of AGW and in others may amplify the AGW impacts.  Addressing the regional impacts of natural variability in combination with possible AGW impacts would increase overall resilience to extreme weather events and climate change/variability.

IMO, the IPCC WG2 should be addressing the social vulnerability aspect in a major way, other than relegating this to a single chapter.   Unfortunately, the IPCC WG2 seems more intent on attributing adverse impacts of extreme weather events and climate variability to AGW (which the IPCC was roundly criticized for in terms of its statements of confidence by the IAC report).