Climate Change: Risk Assessment

Over the past year, Snowchange Co-op has participated in a major new “Climate Change: Risk Assessment”. We contributed the Arctic and Indigenous materials and participated in the write-up and workshops leading to the launch of the report today.

The main highlights are:


A climate change risk assessment must consider at least three areas: the future pathway of global emissions; the direct risks arising from the climate’s response to those emissions; and the risks arising from the interaction of climate change with complex human systems. Each of these areas contains large uncertainties. From our assessment, we draw the following conclusions about the most significant risks.

EMISSIONS: Without increased political commitment and an acceleration of technological innovation, global emissions are likely to follow a medium to high pathway: continuing to increase for the next few decades, and then levelling off or decreasing gradually.

• Current policies and plans for major countries and regions are, in aggregate, consistent with a medium to high emissions pathway, with emissions continuing to increase over the next few decades.

• The technological challenges to achieving a low emissions pathway are substantial, and are not being adequately addressed at present. Without an acceleration of innovation in energy technology and energy systems – including wind and solar with storage, nuclear, biofuel, petroleum-free passenger transport, carbon storage, and large-scale energy efficiency – the likelihood of following a pathway in which emissions fall rapidly and approach zero by late in the century is very low.

• High emissions pathways in which emissions continue to increase throughout the century cannot be ruled out, given the potential for extraction of large new coal reserves, as well as oil shale and methane hydrates.

• The climate responds to cumulative emissions, so any pathway that does not bring emissions close to zero will result in risk continually increasing over time.

DIRECT RISKS: The risks of climate change are non-linear: while average conditions may change gradually, the risks can increase rapidly. On a high emissions pathway, the probability of crossing thresholds beyond which the inconvenient may become intolerable will increase over time.

• For any emissions pathway, a wide range of global temperature increases is possible. On all but the lowest emissions pathways, a rise of more than 2°C is likely in the latter half of this century. On a medium-high emissions pathway (RCP61), a rise of more than 4°C appears to be as likely as not by 2150. On the highest emissions pathway (RCP8.5), a rise of 7°C is a very low probability at the end of this century, but appears to become more likely than not during the course of the 22nd century. A rise of more than 10°C over the next few centuries cannot be ruled out.

• Humans have limited tolerance for heat stress. In the current climate, safe climatic conditions for work are already exceeded frequently for short periods in hot countries, and heat waves already cause fatalities. In future, climatic conditions could exceed potentially lethal limits of heat stress even for individuals resting in the shade. The probability of exposed individuals experiencing such conditions in a given year starts to become significant for a global temperature rise of around 5°C, and could exceed 50% for a global temperature rise of around 7°C, in hot areas such as northern India, southeastern China, and southeastern USA.

• Crops have limited tolerance for high temperatures. When critical thresholds are exceeded, yields may be drastically reduced. The probability of crossing such thresholds in a given year, for studied examples of maize in the Midwestern US and rice in southern China, appears to rise from near zero at present, to become increasingly significant with global temperature rise of more than 2°C, and in the worst cases to reach somewhere in the region of 25% (maize) and 75% (rice) respectively with global temperature rise of around 4-5°C.Biophysical limits on the extent to which such tolerance thresholds can be raised may be an important constraint on adaptation. This is one reason why high degrees of climate change could pose very large risks to global food security.

• Thresholds for water stress are largely arbitrary, but thresholds of ‘moderate’, ‘chronic’ and ‘extreme’ water shortage are widely used, based on per capita availability. The number of people exposed to extreme water shortage is projected to double, globally, by mid century due to population growth alone. Climate change could increase the risk in some regions: for example, on a high emissions pathway, the probability of the Tigris – Euphrates river basin falling into extreme water shortage could rise significantly after 2030, reaching close to 100% by 2070.

• In South and East Asia, climate change may slightly offset otherwise increasing risks of water stress, while increasing the risk of flooding. On a high emissions pathway, what is now a ‘30-year flood’ could become three times more frequent in the Yellow River and Indus basins, and six times more frequent in the Ganges basin, over the course of the century, on a central estimate. In the worst case for those three river basins, such a flood could be in the region of ten times more frequent by the end of the century.

• On a high emissions pathway, the incidence of extreme drought affecting cropland could increase by about 50% in the US and South Asia, double globally, and triple in southern Africa, over the course of the century under central estimates. The uncertainties around these central estimates are large: for the US and South Asia, in the best case, drought incidence could halve; in the worst case, it could increase by three or four times.

• With 1m of global sea level rise, the probability of what is now a ‘100-year flood event’ becomes about 40 times more likely in Shanghai, 200 times more likely in New York, and 1000 times more likely in Kolkata. Defences can be upgraded to maintain the probability of a flood at a constant level, but this will be expensive, and the losses from flooding will still increase, as the floods that do occur will have greater depth. Thresholds of adaptation beyond which ‘retreat’ from the sea may become more feasible than further increases in flood protection are not well defined, but the most significant limits may be sociopolitical rather than economic or technological.

• Climate models suggest that global sea level rise is unlikely to exceed 1m this century, and that a plausible worst-case scenario could result in an increase of several metres by the end of the 22nd century. However, due to inertia in the climate system, with a sustained global temperature rise of 2°C the global sea level may be committed to rise by some 10-15m as ice-sheets gradually melt, but whether this will take hundreds of years or thousands of years is deeply uncertain.

• Many elements of the climate system are capable of abrupt or irreversible change. Changes to monsoons or to ocean circulation patterns, die-back of tropical forests, and the release of carbon from permafrost or sub-sea methane hydrates could all cause large-scale disruption of the climate. The probabilities of such changes are not well known, but are they expected to increase as the global temperature rises.

SYSTEMIC RISKS: The risks of climate change are systemic. The greatest risks may arise from the interaction of the climate with complex human systems such as global food markets, governance arrangements within states, and international security.

• As climate change increases the frequency of extreme weather events, preliminary analysis suggests what was a ‘1 in 100 year’ shock to global food production in the latter half of the 20th century may have become three times more likely by mid-century. If policy and market responses amplify rather than mitigate the shock, a plausible worst-case scenario in the present day could produce unprecedented price spikes on the global market, with a trebling of the prices of the worst-affected grains, compared to current levels.

• Climate change has already increased the probability of extreme events such as the Russian heat wave of 2010, and the Syrian drought of 2007-2011. These events have contributed to unrest and conflict, in combination with other factors such as food export restrictions, existing resource stress, poor governance and state fragility. At low degrees of climate change, further such risks are most likely to arise in regions where climate change is reducing already stressed resources at the same time as high rates of population growth are increasing demand.

• Security risks at high degrees of climate change seem likely to be of a different order of magnitude. Extreme water stress, and competition for productive land, could both become sources of conflict. Migration from some regions may become more a necessity than a choice, and could take place on a historically unprecedented scale. It seems likely that the capacity of the international community for humanitarian assistance would be overwhelmed. The risks of state failure could rise significantly, affecting many countries simultaneously, and even threatening those that are currently considered developed and stable. The expansion of ungoverned territories would in turn increase the risks of terrorism. The temptation for states or other actors to take unilateral steps toward climate geoengineering would be significant, and could become a further source of conflict.

VALUE: Valuing these risks is essentially a subjective exercise.

• Standard economic estimates of the global costs of climate change are wildly sensitive both to assumptions about the science, and to judgments about the value of human life. They are also likely to be systematically biased towards underestimation of risk, as they tend to omit a wide range of impacts that are difficult to quantify.

• Even if economic costs could be estimated accurately, their sum total would not be a good measure of the risks of climate change. Some of the greatest tragedies of the last century had a negligible impact on global GDP. Some of the greatest risks of climate change may be similarly non-monetary.

• Any valuation of the risks of climate change will involve subjective judgments, most notably with regard to the importance attached to the wellbeing of future generations. Such judgments should be made transparently, so that they may be publicly debated.


There is much that we can do to improve our assessment of climate change risk. This is an opportunity, as it can better inform decisions on risk reduction.

Our recommendations on risk assessment are: apply the right principles; broaden participation in the process; and report to the highest decision-making authorities.

Apply the principles of risk assessment. These include: • Assess risks in relation to objectives, or interests. Start from an understanding of what it is that we wish to avoid; then assess its likelihood.

• Identify the biggest risks. Focus on finding out more about worst-case scenarios in relation to long- term changes, as well as short-term events.

• Consider the full range of probabilities, bearing in mind that a very low probability may correspond to a very high risk, if the impact is catastrophic.

• Use the best available information, whether this is proven science, or expert judgment. A best estimate is usually better than no estimate at all.

• Take a holistic view. Assess systemic risks, as well as direct risks. Assess risks across the full range of space and time affected by the relevant decisions.

• Be explicit about value judgments. Recognize that they are essentially subjective, and present them transparently so that they can be subject to public debate.

Risk assessments need to be made on a regular and consistent basis, so that in areas of uncertainty, any changes or trends in expert judgment are clearly visible over time. This could be facilitated by the identification and use of a consistent set of indicators in each of the three areas of risk assessment described above.

Broaden participation in the risk assessment process. Different participants are important to different stages of the process:

Defining objectives: Leaders and decision-makers have a role at the beginning, in defining the objectives and interests against which risks should be assessed.

Information gathering: Scientists have the lead role in understanding climate change and its direct impacts. Experts in politics, technology, economics, and other disciplines can provide information relevant to the future of global emissions, and the indirect impacts of climate change as it interacts with human systems.

• Risk assessment: Whereas information gathering may collect whatever is useful or interesting, risk assessment interrogates that evidence in relation to defined objectives and according to a specific set of principles. Separating these tasks may allow both to be carried out more effectively. Climate change risk assessments should involve not only scientists, but also experts in risk, who may be drawn from fields such as defence, intelligence, insurance, and public health.

Report to the highest decision-making authorities. A risk assessment aims to inform those with the power to reduce or manage the risk. Assessments of specific, local, or sectoral risks of climate change may be directed at those with specific, local or sectoral responsibility. Assessments of the risk of climate change as a whole should report directly to those with responsibility for governance as a whole. At the national level, this means the head of government, the cabinet, or the national security council. At the global level, it means institutions where heads of government meet to make decisions.

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