1.2.3.4 Uncertainty and risk

Uncertainty in knowledge is an important aspect in the implementation of Article 2, whether it is assessing future GHG emissions or the severity of climate change impacts and regional changes, evaluating these impacts over many generations, estimating mitigation costs or evaluating the level of mitigation action needed to reduce risk. Notwithstanding these uncertainties, mitigation will reduce the risk of both global mean and regional changes and the risk of abrupt changes in the climate system (see Chapter 2, Section 2.3).

There may be risks associated with rapid and/or abrupt changes in the climate and the climate system as a result of human interference (Solomon et al., 2007; IPCC, 2007b, Chapter 19 Tables 19.1 and 19.3.5-7). These include changes in climate variability (El Nino Southern Oscillation, monsoons); a high likelihood that warming will lead to an increase in the risk of many extreme events, including floods, droughts, heat waves and fires, with increasing levels of adverse impacts; a risk that a 1–2ºC sustained global warming (versus the temperature at present) would lead to a commitment to a large sea-level rise due to at least the partial deglaciation of both ice sheets; an uncertain risk of a shutdown of the North Atlantic Meridional Overturning Circulation; a large increase in the intensity of tropical cyclones with increasing levels of adverse impacts as temperatures increase; the risk that positive feedbacks from warming may cause the release of CO₂ or methane (CH₄) from the terrestrial biosphere and soils (IPCC, 2007b, Chapter 19 Tables 19.1 and 19.3.5-7). In the latter case, a positive climate–carbon cycle feedback would reduce the land and ocean uptake of CO₂, implying a reduction of the allowable emissions required to achieve a given atmospheric CO₂ stabilization level (Meehl et al., 2007, Executive Summary).

1.2.3.5 Irreversibility

Irreversibility is an important aspect of the climate change issue, with implications for mitigation and adaptation responses. The response of the climate system to anthropogenic forcing is likely to be irreversible over human time scales, and much of the damage is likely to be irreversible even over longer time scales. Mitigation and adaptation will often require investments involving sunk (irreversible) costs in new technologies and practices (Sections 2.2.3, 11.6.5; IPCC, 2007b, Chapter 17). Decision-makers will need to take into account these environmental, socio-economic and technological irreversibilities in deciding on the timing and scale of mitigation action.