Executive summary
Vulnerability to specific impacts of climate change will be most severe when and where they are felt together with stresses from other sources [20.3, 20.4, 20.7, Chapter 17 Section 17.3.3] (very high confidence).
Non-climatic stresses can include poverty, unequal access to resources, food security, environmental degradation and risks from natural hazards [20.3, 20.4, 20.7, Chapter 17 Section 17.3.3]. Climate change itself can, in some places, produce its own set of multiple stresses; total vulnerability to climate change, per se, is greater than the sum of vulnerabilities to specific impacts in these cases [20.7.2].
Efforts to cope with the impacts of climate change and attempts to promote sustainable development share common goals and determinants including access to resources (including information and technology), equity in the distribution of resources, stocks of human and social capital, access to risk-sharing mechanisms and abilities of decision-support mechanisms to cope with uncertainty [20.3.2, Chapter 17 Section 17.3.3, Chapter 18 Sections 18.6 and 18.7] (very high confidence). Nonetheless, some development activities exacerbate climate-related vulnerabilities [20.8.2, 20.8.3] (very high confidence).
It is very likely that significant synergies can be exploited in bringing climate change to the development community and critical development issues to the climate-change community [20.3.3, 20.8.2, 20.8.3]. Effective communication in assessment, appraisal and action are likely to be important tools, both in participatory assessment and governance as well as in identifying productive areas for shared learning initiatives. Despite these synergies, few discussions about promoting sustainability have thus far explicitly included adapting to climate impacts, reducing hazard risks and/or promoting adaptive capacity [20.4, 20.5, 20.8.3].
Climate change will result in net costs into the future, aggregated across the globe and discounted to today; these costs will grow over time [20.6.1, 20.6.2] (very high confidence).
More than 100 estimates of the social cost of carbon are available. They run from US$-10 to US$+350 per tonne of carbon. Peer-reviewed estimates have a mean value of US$43 per tonne of carbon with a standard deviation of US$83 per tonne. Uncertainties in climate sensitivity, response lags, discount rates, the treatment of equity, the valuation of economic and non-economic impacts and the treatment of possible catastrophic losses explain much of this variation including, for example, the US$310 per tonne of carbon estimate published by Stern (2007). Other estimates of the social cost of carbon span at least three orders of magnitude, from less than US$1 per tonne of carbon to over US$1,500 per tonne [20.6.1]. It is likely that the globally-aggregated figures from integrated assessment models underestimate climate costs because they do not include significant impacts that have not yet been monetised [20.6.1, 20.6.2, 20.7.2, 20.8, Chapter 17 Section 17.2.3, Chapter 19]. It is virtually certain that aggregate estimates mask significant differences in impacts across sectors and across regions, countries and locally [20.6, 20.7, 20.8, Chapter 17 Section 17.3.3]. It is virtually certain that the real social cost of carbon and other greenhouse gases will rise over time; it is very likely that the rate of increase will be 2% to 4% per year [20.6, 20.7]. By 2080, it is likely that 1.1 to 3.2 billion people will be experiencing water scarcity (depending on scenario); 200 to 600 million, hunger; 2 to 7 million more per year, coastal flooding [20.6.2].
Reducing vulnerability to the hazards associated with current and future climate variability and extremes through specific policies and programmes, individual initiatives, participatory planning processes and other community approaches can reduce vulnerability to climate change [20.8.1, 20.8.2, Chapter 17 Sections 17.2.1, 17.2.2 and 17.2.3] (high confidence). Efforts to reduce vulnerability will be not be sufficient to eliminate all damages associated with climate change [20.5, 20.7.2, 20.7.3] (very high confidence).
Climate change will impede nations’ abilities to achieve sustainable development pathways as measured, for example, by long-term progress towards the Millennium Development Goals [20.7.1] (very high confidence).
Over the next half-century, it is very likely that climate change will make it more difficult for nations to achieve the Millennium Development Goals for the middle of the century. It is very likely that climate change attributed with high confidence to anthropogenic sources, per se, will not be a significant extra impediment to nations reaching their 2015 Millennium Development Targets since many other obstacles with more immediate impacts stand in the way [20.7.1].
Synergies between adaptation and mitigation measures will be effective until the middle of this century (high confidence), but even a combination of aggressive mitigation and significant investment in adaptive capacity could be overwhelmed by the end of the century along a likely development scenario [20.7.3, Chapter 18 Sections 18.4, 18.7, Chapter 19] (high confidence).
Until around 2050, it is likely that global mitigation efforts designed to cap effective greenhouse gas concentrations at 550 ppm would benefit developing countries significantly, regardless of whether climate sensitivity turns out to be high or low and especially when combined with enhanced adaptation. Developed countries would also likely see significant benefits from an adaptation-mitigation intervention portfolio, especially for high climate sensitivities and in sectors and regions that are already showing signs of being vulnerable. However, by 2100, climate change will likely produce significant impacts across the globe, even if aggressive mitigation were implemented in combination with significantly enhanced adaptive capacity [20.7.3].