4. What are the likely consequences of climate change?

4.1. Has climate change already started to affect us?
      4.1.1. Natural systems
      4.1.2. Human systems

4.2. How potentially could climate change affect us?
     4.2.1. Natural systems
     4.2.2. Human systems

4.3. What other major impacts could climate change possibly have?
     4.3.1. Could climate change have major consequences?
     4.3.2. What is the risk of large-scale and possibly irreversible impacts?
     4.3.3. What are some examples of the projected changes and their impacts?

4.4. What should be done?
     4.4.1. Who will be affected?
     4.4.2. How can vulnerability be reduced?

4.1. Has climate change already started to affect us?

4.1.1. Natural systems
4.1.2. Human systems

4.1.1. Has climate change already affected natural systems?

"Recent Regional Climate Changes, particularly Temperature Increases, have already Affected Many Physical and Biological Systems

Available observational evidence indicates that regional changes in climate, particularly increases in temperature, have already affected a diverse set of physical and biological systems in many parts of the world.

Documented Climate
Change Impacts

Figure SPM-1

Examples of observed changes include shrinkage of glaciers, thawing of permafrost, later freezing and earlier break-up of ice on rivers and lakes, lengthening of mid- to high-latitude growing seasons, poleward and altitudinal shifts of plant and animal ranges, declines of some plant and animal populations, and earlier flowering of trees, emergence of insects, and egg-laying in birds (see Figure SPM-1). Associations between changes in regional temperatures and observed changes in physical and biological systems have been documented in many aquatic, terrestrial, and marine environments. [2.1, 4.3, 4.4, 5.7, and 7.1] Links...

The studies mentioned above and illustrated in Figure SPM-1 were drawn from a literature survey, which identified long-term studies, typically 20 years or more, of changes in biological and physical systems that could be correlated with regional changes in temperature.5 In most cases where changes in biological and physical systems were detected, the direction of change was that expected on the basis of known mechanisms. The probability that the observed changes in the expected direction (with no reference to magnitude) could occur by chance alone is negligible. In many parts of the world, precipitation-related impacts may be important. At present, there is a lack of systematic concurrent climatic and biophysical data of sufficient length (2 or more decades) that are considered necessary for assessment of precipitation impacts.

Factors such as land-use change and pollution also act on these physical and biological systems, making it difficult to attribute changes to particular causes in some specific cases. However, taken together, the observed changes in these systems are consistent in direction and coherent across diverse localities and/or regions (see Figure SPM-1) with the expected effects of regional changes in temperature. Thus, from the collective evidence, there is high confidence6 that recent regional changes in temperature have had discernible impacts on many physical and biological systems."

Source & © : IPCC TAR SPM of WG II pages 3-4

4.1.2. Has climate change already affected human systems?

"There are Preliminary Indications that Some Human Systems have been Affected by Recent Increases in Floods and Droughts

There is emerging evidence that some social and economic systems have been affected by the recent increasing frequency of floods and droughts in some areas. However, such systems are also affected by changes in socioeconomic factors such as demographic shifts and land-use changes. The relative impact of climatic and socioeconomic factors are generally difficult to quantify. [4.6 and 7.1]" Links...

Source & © : IPCC TAR SPM of WG II page 4

4.2. How potentially could climate change affect us?

4.2.1. Natural systems
4.2.2. Human systems

4.2.1. How potentially could climate change affect natural systems?

"Natural Systems are Vulnerable to Climate Change, and Some will be Irreversibly Damaged

Natural systems can be especially vulnerable to climate change because of limited adaptive capacity, and some of these systems may undergo significant and irreversible damage.

Reasons for Concern

Figure SPM-2

Natural systems at risk include glaciers, coral reefs and atolls, mangroves, boreal and tropical forests, polar and alpine ecosystems, prairie wetlands, and remnant native grasslands. While some species may increase in abundance or range, climate change will increase existing risks of extinction of some more vulnerable species and loss of biodiversity. It is well-established6 that the geographical extent of the damage or loss, and the number of systems affected, will increase with the magnitude and rate of climate change (see Figure SPM-2). [4.3 and 7.2.1]" Links...

Source & © : IPCC TAR SPM of WG II pages 4-5

4.2.2. How potentially could climate change affect human systems?

"Many Human Systems are Sensitive to Climate Change, and Some are Vulnerable

Human systems that are sensitive to climate change include mainly water resources; agriculture (especially food security) and forestry; coastal zones and marine systems (fisheries); human settlements, energy, and industry; insurance and other financial services; and human health. The vulnerability of these systems varies with geographic location, time, and social, economic, and environmental conditions.[4.1, 4.2, 4.3, 4.4, 4.5, 4.6, and 4.7] Links...

• A general reduction in potential crop yields in most tropical and sub-tropical regions for most projected increases in temperature [4.2]
• A general reduction, with some variation, in potential crop yields in most regions in mid-latitudes for increases in annual-average temperature of more than a few °C [4.2]
• Decreased water availability for populations in many water-scarce regions, particularly in the sub-tropics [4.1]
• An increase in the number of people exposed to vector-borne (e.g., malaria) and water-borne diseases (e.g., cholera), and an increase in heat stress mortality
  [4.7]
• A widespread increase in the risk of flooding for many human settlements (tens of millions of inhabitants in settlements studied) from both increased heavy precipitation events and sea-level rise [4.5]
• Increased energy demand for space cooling due to higher summer temperatures.
  [4.5] Links...

• Increased potential crop yields in some regions at mid-latitudes for increases in temperature of less than a few °C [4.2]
• A potential increase in global timber supply from appropriately managed forests
  [4.3]
• Increased water availability for populations in some water-scarce regions—for example, in parts of southeast Asia [4.1]
• Reduced winter mortality in mid- and high-latitudes [4.7]
• Reduced energy demand for space heating due to higher winter temperatures"
  [4.5] Links...

Source & © : IPCC TAR SPM of WG II pages 5-6

4.3. What other major impacts could climate change possibly have?

4.3.1. Could climate change have major consequences?
4.3.2. What is the risk of large-scale and possibly irreversible impacts?
4.3.3. What are some examples of the changes and their impacts?

4.3.1. Could climate change have major consequences?

"Projected Changes in Climate Extremes could have Major Consequences

The vulnerability of human societies and natural systems to climate extremes is demonstrated by the damage, hardship, and death caused by events such as droughts, floods, heat waves, avalanches, and windstorms. While there are uncertainties attached to estimates of such changes, some extreme events are projected to increase in frequency and/or severity during the 21st century due to changes in the mean and/or variability of climate, so it can be expected that the severity of their impacts will also increase in concert with global warming (see Figure SPM-2). [3.5, 4.6, 6, and
7.2.4] Links...

Conversely, the frequency and magnitude of extreme low temperature events, such as cold spells, is projected to decrease in the future, with both positive and negative impacts. The impacts of future changes in climate extremes are expected to fall disproportionately on the poor. Some representative examples of impacts of these projected changes in climate variability and climate extremes are presented in Table SPM-1."

Source & © : IPCC TAR SPM of WG II page 6

4.3.2. What is the risk of large-scale and possibly irreversible impacts?

"The Potential for Large-Scale and Possibly Irreversible Impacts Poses Risks that have yet to be Reliably Quantified

Projected climate changes7 during the 21st century have the potential to lead to future large-scale and possibly irreversible changes in Earth systems resulting in impacts at continental and global scales. These possibilities are very climate scenario-dependent and a full range of plausible scenarios has not yet been evaluated. Examples include significant slowing of the ocean circulation that transports warm water to the North Atlantic, large reductions in the Greenland and West Antarctic Ice Sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments. The likelihood of many of these changes in Earth systems is not well-known, but is probably very low; however, their likelihood is expected to increase with the rate, magnitude, and duration of climate change (see Figure SPM-2). [3.5, 5.7, and
7.2.5] Links...

If these changes in Earth systems were to occur, their impacts would be widespread and sustained. For example, significant slowing of the oceanic thermohaline circulation would impact deep-water oxygen levels and carbon uptake by oceans and marine ecosystems, and would reduce warming over parts of Europe. Disintegration of the West Antarctic Ice Sheet or melting of the Greenland Ice Sheet could raise global sea level up to 3 m each over the next 1,000 years8 , submerge many islands, and inundate extensive coastal areas. Depending on the rate of ice loss, the rate and magnitude of sea-level rise could greatly exceed the capacity of human and natural systems to adapt without substantial impacts. Releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments, induced by warming, would further increase greenhouse gas concentrations in the atmosphere and amplify climate change." [3.5, 5.7, and 7.2.5] Links...

Source & © : IPCC TAR SPM of WG II page 6

4.3.3. What are some examples of the projected changes and their impacts?

Source & © : IPCC TAR SPM of WG II page 7

4.4. What should be done?

4.4.1. Who will be affected?
4.4.2. How can vulnerability be reduced?

"Adaptation is a Necessary Strategy at All Scales to Complement Climate Change Mitigation Efforts

Adaptation has the potential to reduce adverse impacts of climate change and to enhance beneficial impacts, but will incur costs and will not prevent all damages. Extremes, variability, and rates of change are all key features in addressing vulnerability and adaptation to climate change, not simply changes in average climate conditions. Human and natural systems will to some degree adapt autonomously to climate change. Planned adaptation can supplement autonomous adaptation, though options and incentives are greater for adaptation of human systems than for adaptation to protect natural systems. Adaptation is a necessary strategy at all scales to complement climate change mitigation efforts.[6] Links...

Experience with adaptation to climate variability and extremes can be drawn upon to develop appropriate strategies for adapting to anticipated climate change. Adaptation to current climate variability and extremes often produces benefits as well as forming a basis for coping with future climate change. However, experience also demonstrates that there are constraints to achieving the full measure of potential adaptation. In addition, maladaptation, such as promoting development in risk-prone locations, can occur due to decisions based on short-term considerations, neglect of known climatic variability, imperfect foresight, insufficient information, and over-reliance on insurance mechanisms." [6] Links...

Source & © : IPCC TAR SPM of WG II pages 6 and 8

4.4.1. Who will be affected?

"Those with the Least Resources have the Least Capacity to Adapt and are the Most Vulnerable

The ability of human systems to adapt to and cope with climate change depends on such factors as wealth, technology, education, information, skills, infrastructure, access to resources, and management capabilities. There is potential for developed and developing countries to enhance and/or acquire adaptive capabilities. Populations and communities are highly variable in their endowments with these attributes, and the developing countries, particularly the least developed countries, are generally poorest in this regard. As a result, they have lesser capacity to adapt and are more vulnerable to climate change damages, just as they are more vulnerable to other stresses. This condition is most extreme among the poorest people. [6.1; see also 5.1.7, 5.2.7, 5.3.5, 5.4.6, 5.6.1, 5.6.2, 5.7, and 5.8.1 for regional-scale information] Links...

Benefits and costs of climate change effects have been estimated in monetary units and aggregated to national, regional, and global scales. These estimates generally exclude the effects of changes in climate variability and extremes, do not account for the effects of different rates of change, and only partially account for impacts on goods and services that are not traded in markets. These omissions are likely to result in underestimates of economic losses and overestimates of economic gains. Estimates of aggregate impacts are controversial because they treat gains for some as canceling out losses for others and because the weights that are used to aggregate across individuals are necessarily subjective.[7.2.2 and 7.2.3] Links...

Notwithstanding the limitations expressed above, based on a few published estimates, increases in global mean temperature9 would produce net economic losses in many developing countries for all magnitudes of warming studied (low confidence6), and losses would be greater in magnitude the higher the level of warming (medium confidence6). In contrast, an increase in global mean temperature of up to a few °C would produce a mixture of economic gains and losses in developed countries (low confidence6), with economic losses for larger temperature increases (medium confidence6). The projected distribution of economic impacts is such that it would increase the disparity in well-being between developed countries and developing countries, with disparity growing for higher projected temperature increases (medium confidence6). The more damaging impacts estimated for developing countries reflects, in part, their lesser adaptive capacity relative to developed countries.[7.2.3]

Further, when aggregated to a global scale, world gross domestic product (GDP) would change by ± a few percent for global mean temperature increases of up to a few °C (low confidence6), and increasing net losses would result for larger increases in temperature (medium confidence6) (see Figure SPM-2). More people are projected to be harmed than benefited by climate change, even for global mean temperature increases of less than a few °C (low confidence6). These results are sensitive to assumptions about changes in regional climate, level of development, adaptive capacity, rate of change, the valuation of impacts, and the methods used for aggregating monetary losses and gains, including the choice of discount rate. [7.2.2 and 7.2.3] Links...

The effects of climate change are expected to be greatest in developing countries in terms of loss of life and relative effects on investment and the economy. For example, the relative percentage damages to GDP from climate extremes have been substantially greater in developing countries than in developed countries." [4.6] Links...

Source & © : IPCC TAR SPM of WG II page 8

4.4.2. How can vulnerability be reduced?

"Adaptation, Sustainable Development, and Enhancement of Equity can be Mutually Reinforcing

Many communities and regions that are vulnerable to climate change are also under pressure from forces such as population growth, resource depletion, and poverty. Policies that lessen pressures on resources, improve management of environmental risks, and increase the welfare of the poorest members of society can simultaneously advance sustainable development and equity, enhance adaptive capacity, and reduce vulnerability to climate and other stresses. Inclusion of climatic risks in the design and implementation of national and international development initiatives can promote equity and development that is more sustainable and that reduces vulnerability to climate change." [6.2] Links...

Source & © : IPCC TAR SPM of WG II page 8