Working Group II

Last updated: October 23, 2014

Introduction

Working Group II (WGII) covers the impacts and vulnerabilities caused by climate change and discusses pathways for adaptation. Delegates from around the world approved the final Summary for Policymakers (SPM) in Yokohama, Japan on March 31. The final summary is available online on the official WGII site. The following overview seeks to provide context for those interested in understanding the broad conclusions of WGII.


Findings of Note: Observed Impacts

WGII provides an overview of the impacts of climate change that we have already observed and highlights areas where society is particularly vulnerable to climate impacts. Significant findings on this topic include:

Climate change is now everywhere. Impacts have been found on every continent. Both the human and natural worlds are feeling the effects, which are consequential and growing. Emphasis on currently occurring impacts has increased since the AR4, which only said that impacts are “emerging.” (more

People everywhere are vulnerable to extreme climate events. The devastating impacts of recent extreme events and extreme weather disasters show that our level of adaptation remains low. The strength and immediacy of this statement has increased since the AR4, which emphasized impacts in the far future more than current impacts.  (more)

Many species have migrated to new locations and changed the timing of their seasonal behavior in response to climate change. This will persist as warming continues, and many species will be unable to move fast enough to adapt. (more)

The impact of warming on global agriculture has been and will continue to be negative. Recent rapid increases in food prices show that markets are sensitive to climate variability. Potential benefits to warming in some localized regions will not be sufficient to offset the negative impacts. (more)

Increasingly, governments are incorporating climate adaptation measures seamlessly into policy frameworks. They are also recognizing and focusing on the most vulnerable populations. In North America, local government is showing the most leadership in adaptation planning. (more)

We’re close to missing our chance to limit warming to 1.5ºC beyond pre-industrial levels. This underscores the need for immediate action if we are to remain below 2ºC, or anywhere close. World leaders have agreed to make 2ºC the goal of international climate negotiations, as beyond this point climate change impacts will begin to be unacceptably severe.  (more)


Findings of Note: Future Impacts

WGII also reviews the state of scientific knowledge on projected impacts of climate change as warming trends continue. These projections reveal the serious disruptions to the functioning of human society that will occur unless the world takes aggressive action to limit greenhouse gas pollution.

Runaway warming would make some regions uninhabitable. A global temperature increase of 4˚C could make normal activities like growing food or working outdoors impossible in many regions where people currently live.  (more

Reducing carbon pollution rapidly and immediately can cut the overall economic damage of climate change by half. This finding puts the costs of climate mitigation into valuable context. Although mitigation will be costly, action now will drastically cut overall costs. (more

Without adaptation, even warming of only 2ºC will negatively affect crop yield. Increasing adaptation may ameliorate these impacts on yield, up to a point. Warming above 4ºC will exceed the ability of adaptation to “keep up” and poses significant risks to food security. (more

Climate change will increase the frequency and severity of many types of extreme weather. In many areas, more frequent intense rainfall events will increase the frequency of flooding. Globally, more people will be exposed to floods and economic losses due to flooding. It’s also likely that presently dry areas will become more severely drought-stricken in a warmer world. (more

Adaptation (dealing with the direct effects of climate changes) and mitigation (reducing emissions to prevent further changes) are both essential to climate policy. Adaptation is unavoidable, as even the most aggressive pollution reduction still won’t eliminate some additional warming. Without mitigation, impacts will grow larger than our capacity to adapt. (more)

Climate change will negatively impact many human systems. High agreement between varying studies and sources of evidence exists for significant impacts on transportation infrastructure, the health sector, and insurance systems. Moderate agreement exists for impacts on many more systems. (more


Observed Impacts and Vulnerabilities

Climate change is now everywhere. Impacts have been found on every continent. Both the human and natural worlds are feeling the effects, which are consequential and growing. Emphasis on currently occurring impacts has increased since the AR4, which only said that impacts are “emerging.”

For example, the AR5 states high risk levels of present climate impacts to human systems including: the incidence and range of disease in Africa; property loss and mortality due to wildfires in North America; and decreased food production and food quality in South America. Medium risk levels are given for many more impacts (Box SPM.2 Table 1). In this respect the AR5 goes into much greater detail than the AR4, and shows greater certainty with respect to observed human impacts.

The AR5 describes the contribution of climate change to impacts on human systems as “major”, which is significant in that the AR5 is circumspect on what constitutes “dangerous” climate change or an unacceptable level of risks or impacts (Box SPM.1). To some extent this is a judgment that world leaders will need to make individually according to their values. The conclusion that some impacts to human systems are major implies that serious consideration of risks and solutions is needed now.

AR5 Quote: “In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Evidence of climate-change impacts is strongest and most comprehensive for natural systems. Some impacts on human systems have also been attributed to climate change, with a major or minor contribution of climate change distinguishable from other influences.[WGII SPM p. 6]

(Note: For summary of observed human impacts, see Box SPM.2 Table 1.)

AR4 Quote: “Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases … A global assessment of data since 1970 has shown it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems.” [AR4]

AR4 Quote: “Other effects of regional climate changes on natural and human environments are emerging, although many are difficult to discern due to adaptation and non-climatic drivers.”  [AR4]


 
People everywhere are vulnerable to extreme climate events. The devastating impacts of recent extreme events and extreme weather disasters show that our level of adaptation remains low. This holds true for countries at “all levels of development,”not just developing countries. The strength and immediacy of this statement has increased since the AR4, which emphasized impacts in the far future more than current impactsFor example, current infrastructure in North America is vulnerable to climate change and requires strengthening adaptation. Without increases in adaptation agricultural production is also projected to experience declines that could be large enough to affect global food security. The AR5 identifies a “significant lack of preparedness” that will need to be addressed to avoid some types of serious climate impacts.

AR5 Quotes: “Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability (very high confidence). Impacts of such climate-related extremes include alteration of ecosystems, disruption of food production and water supply, damage to infrastructure and settlements, morbidity and mortality, and consequences for mental health and human well-being. For countries at all levels of development, these impacts are consistent with a significant lack of preparedness for current climate variability in some sectors.” [WGII SPM p. 3, line 30]

Limited evidence indicates a gap between global adaptation needs and the funds available for adaptation (medium confidence). There is a need for a better assessment of global adaptation costs, funding, and investment. Studies estimating the global cost of adaptation are characterized by shortcomings in data, methods, and coverage (high confidence).[WGII SPM p. 24]

(Note that the sections below are specific to North America)

“Much of North American infrastructure is currently vulnerable to extreme weather events and, unless investments are made to strengthen them, would be more vulnerable to climate change (medium confidence).” [WGII Chapter 26 p. 5]

“Projected increases in temperature, reductions in precipitation in some regions, and increased frequency of extreme events would result in net productivity declines in major North American crops by the end of the 21st Century without adaptation, although the rate of decline varies by model and scenario, and some regions, particularly in the north, may benefit (very high confidence)… Given that North America is a significant source of global food supplies, projected productivity declines here may affect global food security (medium confidence). At 2°C, adaptation has high potential to off-set projected declines in yields for many crops, and many strategies offer mitigation co-benefits; but effectiveness of adaptation would be reduced at 4°C (high confidence).” [WGII Chapter 26 p. 4]

AR4 Quotes: “The resilience of many ecosystems is likely to be exceeded this century by an unprecedented combination of climate change, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification), and other global change drivers (e.g., land-use change, pollution, over-exploitation of resources).”  [AR4]

“Poor communities can be especially vulnerable, in particular those concentrated in high-risk areas. They tend to have more limited adaptive capacities, and are more dependent on climate-sensitive resources such as local water and food supplies.”  [AR4]


 
Many species have migrated to new locations and changed the timing of their seasonal behavior in response to climate change. Plants and animals are shifting their ranges towards the poles and to higher altitudes in order to remain in climate conditions to which they are accustomed. As spring arrives earlier and winter arrives later in many regions, species are altering the timing of their seasonal activities (such as blooming or migration) accordingly. These shifts have caused the USDA to alter its established hardiness zones, which advise farmers and gardeners as to which plants can be planted where, and when.

For more on how seasonal shifts have affected gardeners, see this Climate Nexus backgrounder. Science Daily and The Ecologist also recently discussed an article in Nature regarding how climate change impacts affect even the most common species and the importance of mitigation strategies.

These species changes will continue in response to climate change, and many species will be unable to alter their range and behaviors at the same rate at which the climate changes. This could put their survival at risk.

AR5 Quotes: “Many plant and animal species have moved their ranges, altered their abundance and shifted their seasonal activities in response to observed climate change over recent decades (high confidence). They are doing so now in many regions and will continue to do so in response to projected future climate change (high confidence). The broad patterns of species and biome shifts towards the poles and higher in altitude in response to a warming climate are well established for periods thousands of years in the past (very high confidence). These general patterns of range shifts have also been observed over the last few decades in some well-studied species groups such as insects and birds and can be attributed to observed climatic changes (high confidence). Interactions between changing temperature, precipitation and land use can sometimes result in range shifts that are downhill or away from the poles. Certainty regarding past species movements in response to changing climate, coupled with projections from a variety of models and studies, provide high confidence that such species movements will be the norm with continued warming.” [WGII Chpt. 4 Summary]

“Many species will be unable to move fast enough during the 21st century to track suitable climates under mid- and high-range rates of climate change (i.e., RCP 4.5, RCP 6.0 and RCP 8.5 scenarios) (medium confidence). The climate velocity (the rate of movement of the climate across the landscape) will exceed the maximum velocity at which many groups of organisms, in many situations, can disperse or migrate, except after mid-century in the RCP 2.6 scenario.” [WGII Chpt. 4 Summary]

(Note that the RCP 2.6 scenario mentioned above is the scenario with the most significant and rapid efforts towards climate change mitigation.)

AR4 Quote: “During the course of this century the resilience of many ecosystems (their ability to adapt naturally) is likely to be exceeded by an unprecedented combination of change in climate, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification) and in other global change drivers (especially land-use change, pollution and over-exploitation of resources), if greenhouse gas emissions and other changes continue at or above current rates (high confidence).” [AR4]


 
The impact of warming on global agriculture has been and will continue to be negative. Recent rapid increases in food prices show that markets are sensitive to climate variability.

Water supply variability, for example, threatens food and livestock production in both irrigated and rainfed systems. Also, recent extremes have caused drops of 25% or more in output from top food producers: a 2009 drought impacted maize and soy in Argentina; a 2010 drought and heat wave impacted Russian wheat production; and severe drought impacted maize production in the U.S. in 2012. Each of these production drops have been followed by rapid price increases.

Potential benefits to warming in some (mostly high-latitude) regions will not be sufficient to offset the negative impacts.

AR5 Quotes: “Based on many studies covering a wide range of regions and crops, negative impacts of climate change on crop yields have been more common than positive impacts (high confidence). The smaller number of studies showing positive impacts relate mainly to high-latitude regions, though it is not yet clear whether the balance of impacts has been negative or positive in these regions (high confidence). Climate change has negatively affected wheat and maize yields for many regions and in the global aggregate (medium confidence)…Since AR4, several periods of rapid food and cereal price increases following climate extremes in key producing regions indicate a sensitivity of current markets to climate extremes among other facts (medium confidence).[WGII SPM p. 7]

“Water demand and use for food and livestock feed production is governed not only by crop management and its efficiency, but also by the balance between atmospheric moisture deficit and soil water supply. Thus, changes in climate (precipitation, temperature, radiation) will affect the water demand of crops grown in both irrigated and rainfed systems.” [WGII Chapter 3.5.2]

“The effects of climate change on crop and food production are evident in several regions of the world (high confidence). Negative impacts of climate trends have been more common than positive ones. Positive trends are evident in some high latitude regions (high confidence). Since AR4, there have been several periods of rapid food and cereal price increases following climate extremes in key producing regions, indicating a sensitivity of current markets to climate extremes among other factors. Several of these climate extremes were made more likely as the result of anthropogenic emissions (medium confidence).” [WGII Chapter 7 Summary]

“Since the AR4 report, international food prices have reversed historical downward trend. Plot shows history of FAO food and cereal price index (composite measures of food prices), with vertical lines indicating events when a top 5 producer of a crop had yields 25% below trend line (indicative of a seasonal climate extreme). Australia is included despite not being a top five producer, because it is an important exporter and the drops were 40% or more below trend line. Prices may have become more sensitive to weather-related supply shortfalls in recent years. At the same time, food prices are increasingly associated with the price of crude oil (blue line), making attribution of price changes to climate difficult.” [WGII Chapter 7 Figure 7-3]

AR4 Quote: “Impacts of climate change will vary regionally but, aggregated and discounted to the present, they are very likely to impose net annual costs which will increase over time as global temperatures increase.” [AR4]



Increasingly, governments are incorporating climate adaptation measures seamlessly into policy frameworks. They are also recognizing and focusing on the most vulnerable populations. In North America, local government is showing the most leadership in adaptation planning. Adaptation efforts already underway include water conservation schemes, saltwater marsh restoration, and modifications to insurance systems. Barriers to adaptation still exist in the form of legal constraints, lack of access to scientific information, and lack of coordination across jurisdictions.

AR5 Quotes: “Engineered and technological adaptation options are still the most common adaptive responses, although there is growing experience of the value for ecosystem-based, institutional, and social measures, including the provision of climate-linked safety nets for those who are most vulnerable. Adaptation measures are increasing and becoming more integrated within wider policy frameworks. Integration, while it remains a challenge, streamlines the adaptation planning and decision making process and embeds climate sensitive thinking in existing and new institutions and organizations… The increasing complexity of adaptation practice means that institutional learning is an important component of effective adaptation (High agreement, robust evidence).” [WGII Chapter 14, p. 2]

(Note that the section below is specific to North America)

“Even though Canada and the US are relatively well-endowed in their capacity to adapt, there are significant constraints on adaptation, with financing being a significant constraint in all three countries. Barriers include legal constraints, lack of coordination across different jurisdictions, leadership, and divergent perceptions about climate change. Although obtaining accurate scientific data was ranked less important by municipalities, an important constraint is lack of access to scientific information and capacity to manage and use it.” [WGII Chapter 26 p. 43]

“Leadership in adaptation is far more evident locally than at other tiers of government in North America… Climate change policies have been motivated by concerns for local economic or energy security and the desire to play leadership roles. Some policies constitute “integrated” strategies (New York) and coordinated participation of multiple municipalities (Vancouver). Sector-specific climate risk management plans have also emerged (e.g., water conservation in Phoenix, US and Regina, Canada; wildfire protection in Kamloops, Canada and Boulder, US). Municipalities affected by the mountain pine beetle have taken many steps toward adaptation, and coastal communities in eastern Canada are investing in saltwater marsh restoration to adapt to rising sea levels. Green roofs, forest thinning and urban agriculture have all been expanding (Chicago, New York, Kamloops, Mexico City), as have flood protection (New Orleans, Chicago), private and governmental insurance policies, safe saving schemes (common in Mexico), air pollution controls (Mexico City), and hazard warning systems.” [WGII Chapter 26 p. 40]

AR4 Quote: “For market and social systems there is considerable adaptation potential, but the economic costs are potentially large, largely unknown and unequally distributed, as is the adaptation potential itself. For biological and geophysical systems, the adaptation potential is much less than in social and market systems, because impacts are more direct and therefore appear more rapidly. A large proportion of the future increase in key vulnerabilities is likely to be recorded first in biological systems (see Chapter 1). This does not mean that key vulnerabilities will not occur in social and market systems. They depend on biological systems, and as ecosystems are affected by mounting stresses from climate change.”  [AR4]


 
We’re close to missing our chance to limit warming to 1.5ºC beyond pre-industrial levels. This underscores the need for immediate action if we are to remain below or anywhere close to 2ºC. World leaders have agreed to make 2ºC the goal of international climate negotiations, on the grounds that beyond this point the impacts of climate change will become unacceptably severe.

This finding is especially relevant to contrarian arguments that moderate warming will be beneficial. We’re already committed to moderate warming. Acting now will be necessary to avoid extreme warming.

AR5 Quote: “Under any plausible scenario for mitigation and adaptation, some degree of risk from residual damages is unavoidable (very high confidence). For example, very few integrated assessment model-based scenarios in the literature demonstrate the feasibility of limiting warming to a maximum of 1.5°C with at least 50% likelihood.” [WGII Chapter 19 p. 6]

AR4 Quote: “The [cost-benefit analysis] literature assessed in this sub-section indicates that initial studies tended to focus on the relationship between mitigation and damages avoided, but our knowledge of this subject is still limited and more research needs to be undertaken. More recently, the literature has begun to focus on the relationship between adaptation and damages avoided. Ultimately, better knowledge about the interaction between adaptation and mitigation actions in terms of damages avoided would be useful. However, such research is at a very rudimentary stage. Moreover, large-scale modeling of adaptation-mitigation feedbacks is needed but still lacking.” [AR4]


Future Risks

Runaway warming would make some regions uninhabitable. A global temperature increase of 4˚C could make normal activities like growing food or working outdoors impossible in many regions where people currently live. A global temperature increase of 7ºC could cause temperatures in some regions to exceed human physiological limits. The AR4 does not mention these risks, but AR5 projects that climate change will increase displacement. Resulting migrations could pose significant disruptions to international politics and economics.

The World Bank issued a 2012 report on the dangers of a world 4ºC warmer, and the AR5 brings additional clarity to the serious impacts that would accompany a temperature rise of this magnitude.

As a further resource, the Age of Consequences study conducted by a diverse group of climate science, policy, and national security experts evaluates the foreign policy and national security implications of global climate change according to three scenarios with increasingly severe impacts. The third scenario looks at what would happen in a world with an average temperature increase of 5.6°C.

Global climate change risks are high to very high with global mean temperature increase of 4°C or more above preindustrial levels in all reasons for concern, and include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, and the combination of high temperature and humidity compromising normal human activities, including growing food or working outdoors in some areas for parts of the year (high confidence).[WGII SPM p. 14]

AR5 Quotes: “Under 4°C warming most of the world land area will be experiencing 4-7°C higher temperatures than the recent past which means that important tipping points for health impacts may be exceeded in many areas of the world during this century, including coping mechanisms for daily temperature/humidity making potentially large areas seasonally uninhabitable for normal human activities, including growing food or working outdoors (high confidence). Exceedance of human physiological limits is projected in some areas for a global warming of 7°C, and in most areas for global warming of 11-12°C (low confidence), a temperature increase that is possible by 2300.” [WGII Chapter 19 p. 26]

“Climate change over the 21st century is projected to increase displacement of people (medium evidence, high agreement). Displacement risk increases when populations that lack the resources for planned migration experience higher exposure to extreme weather events, in both rural and urban areas, particularly in developing countries with low income. Changes in migration patterns can be responses to both extreme weather events and longer-term climate variability and change, and migration can also be an effective adaptation strategy. [WGII SPM p.20]

AR4 Quote: “Studies in temperate areas have shown that climate change is projected to bring some benefits, such as fewer deaths from cold exposure. Overall it is expected that these benefits will be outweighed by the negative health effects of rising temperatures worldwide, especially in developing countries.”  [AR4]


 
Reducing carbon pollution rapidly and immediately can cut the overall economic damage of climate change by half. Delaying action then rapidly reducing emissions would be less effective in terms of avoiding negative impacts. This finding puts the costs of climate mitigation into valuable context. While mitigation will be costly, action now will drastically cut overall costs.

Economic losses start between 0.2% and 2% of global income with warming of ~2°C, and accelerate with warming from there. This is a step up in detail from the AR4, but the report acknowledges that these loss estimates are incomplete and that costs are more likely to above this range rather than below.

AR5 Quote: “Figure 19-6 highlights results from three studies that estimated the global avoided impacts for multiple sectors when global average temperature is limited to 2°C rather than following scenarios with no mitigation, such as the SRES A1B or A1FI baseline scenarios in which global average temperature reaches 4 and 5.6°C respectively. The studies isolate the effects of climate change by using

common socioeconomic assumptions in mitigation and baseline scenarios. Overall, sector-specific impacts were reduced by 20-80%, with aggregate global economic damages reduced by about one half…. The timing of emissions reductions strongly affects impacts. In general fewer impacts can be avoided when mitigation is delayed, because there are limits to how fast emissions can be reduced subsequently to compensate for the delay.” [WGII Chapter 19 p. 47]

Economic impact estimates completed over the past 20 years vary in their coverage of subsets of economic sectors and depend on a large number of assumptions, many of which are disputable, and many estimates do not account for catastrophic changes, tipping points, and many other factors. With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income (±1 standard deviation around the mean) (medium evidence, medium agreement). Losses are more likely than not to be greater, rather than smaller, than this range (limited evidence, high agreement). Additionally, there are large differences between and within countries. Losses accelerate with greater warming (limited evidence, high agreement), but few quantitative estimates have been completed for additional warming around 3°C or above. Estimates of the incremental economic impact of emitting carbon dioxide lie between a few dollars and several hundreds of dollars per tonne of carbon (robust evidence, medium agreement). Estimates vary strongly with the assumed damage function and discount rate.[WGII SPM p. 19]

 “Adaptation and mitigation choices in the near-term will affect the risks of climate change throughout the 21st century (high confidence)…The benefits of adaptation and mitigation occur over different but overlapping timeframes. Projected global temperature increase over the next few decades is similar across emission scenarios. During this near-term period, risks will evolve as socioeconomic trends interact with the changing climate. Societal responses, particularly adaptations, will influence near-term outcomes.[WGII SPM p. 10]

The overall risks of climate change impacts can be reduced by limiting the rate and magnitude of climate change. Risks are reduced substantially under the assessed scenario with the lowest temperature projections (RCP2.6 – low emissions) compared to the highest temperature projections (RCP8.5 – high emissions), particularly in the second half of the 21st century (very high confidence). Reducing climate change can also reduce the scale of adaptation that might be required. Under all assessed scenarios for adaptation and mitigation, some risk from adverse impacts remains (very high confidence).[WGII SPM p. 14]

AR4 Quote: “A wide array of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change. There are barriers, limits and costs, but these are not fully understood…  Sustainable development can reduce vulnerability to climate change, and climate change could impede nations’ abilities to achieve sustainable development pathways.” [AR4]


 
Without adaptation, even warming of only 2ºC will negatively affect crop yield. Increasing adaptation may ameliorate these impacts on yield, up to a point. Warming above 4ºC will exceed the ability of adaptation to “keep up” and poses significant risks to food security. At the same time, demand for food crops will increase and exacerbate risks.

These predictions refute the AR4’s earlier estimate that low-level warming could be beneficial to food production overall. The AR5 acknowledges that increasing CO2 will increase the growth rates of some food crops, but it will also aid the growth of invasive weeds and may reduce the effectiveness of some herbicides. Furthermore, limited availability of nutrients and water could prevent some crops from making full use of the benefits of additional CO2.

AR5 Quotes:

For the major crops (wheat, rice, and maize) in tropical and temperate regions, climate change without adaptation is projected to negatively impact production for local temperature increases of 2°C or more above late-20th-century levels, although individual locations may benefit (medium confidence). Projected impacts vary across crops and regions and adaptation scenarios, with about 10% of projections for the period 2030-2049 showing yield gains of more than 10%, and about 10% of projections showing yield losses of more than 25%, compared to the late 20th century. After 2050 the risk of more severe yield impacts increases and depends on the level of warming…Climate change is projected to progressively increase inter-annual variability of crop yields in many regions. These projected impacts will occur in the context of rapidly rising crop demand.[WG II SPM p. 17-18]

Global climate change risks are high to very high with global mean temperature increase of 4°C or more above preindustrial levels in all reasons for concern, and include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, and the combination of high temperature and humidity compromising normal human activities, including growing food or working outdoors in some areas for parts of the year (high confidence).[WGII SPM p. 14]

“Crop yields remain the most well studied aspect of food security impacts from climate change, with many projections published since AR4. These newer studies confirm many of the patterns identified in AR4, such as negative yield impacts for all crops past 3°C of local warming without adaptation, even with benefits of high CO2 and rainfall.” [WGII Chpt, 7.4.1]

“Changes in climate and CO2 concentration will enhance the distribution and increase the competiveness of agronomically important and invasive weeds (medium confidence). Rising CO2 may reduce the effectiveness of some herbicides (low confidence). The effects of climate change on disease pressure on food crops is uncertain, with evidence pointing to changed geographical ranges of pests and diseases but less certain changes in disease intensity (low confidence).” [WGII Chapter 7 Summary]

“FACE [Free-air Carbon Dioxide Enrichment] studies have shown that the impact of elevated CO2 varies according to temperature and availability of water and nutrients, although the strong geographical bias of FACE studies towards temperate zones limits the strength of this evidence. FACE studies have shown that CO2 enhancment is limited under both low and high temperature.” [WGII Chapter 7 p. 14]

AR4 Quotes: “Crop productivity is projected to increase slightly at mid- to high latitudes for local mean temperature increases of up to 1-3°C depending on the crop, and then decrease beyond that in some regions.

At lower latitudes, especially seasonally dry and tropical regions, crop productivity is projected to decrease for even small local temperature increases (1-2°C), which would increase the risk of hunger.

Globally, the potential for food production is projected to increase with increases in local average temperature over a range of 1-3°C, but above this it is projected to decrease.”  [AR4]


 
Climate change will increase the frequency and severity of many types of extreme weather. In many areas, more frequent intense rainfall events will increase the frequency of flooding. Globally, more people will be exposed to floods and economic losses due to flooding will grow. It’s also likely that presently dry areas will become more severely drought-stricken in a warmer world.

AR5 Quotes: “Climate change will lead to increased frequency, intensity and/or duration of extreme weather events such as heavy rainfall, warm spells and heat events, drought, intense storm surges and associate sea-level rise.” [WGII Chapter 8, p. 17]

So far there are no widespread observations of changes in flood magnitude and frequency due to anthropogenic climate change, but projections imply variations in the frequency of floods (medium agreement, limited evidence). Flood hazards are projected to increase in parts of south, southeast and northeast Asia, tropical Africa, and South America (medium agreement, limited evidence). Since the mid-20th century, socio-economic losses from flooding have increased mainly due to greater exposure and vulnerability (high confidence). Global flood risk will increase in the future partly due to climate change (medium agreement, limited evidence).” [WGII Chapter 3 p. 2]

“Climate change is likely to increase the frequency of meteorological droughts (less rainfall) and agricultural droughts (less soil moisture) in presently dry regions by the end of this century under the RCP8.5 scenario (medium confidence) [WGI Ch12]. This is likely to increase the frequency of short hydrological droughts (less surface water and groundwater) in these regions (medium agreement, medium evidence) [3.4.8]. Projected changes in the frequency of droughts longer than 12 months are more uncertain, because these depend on accumulated precipitation over long periods.” WGII Chapter 3, p. 3]

AR4 Quote: “A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts. As there are a number of climatic and non-climatic drivers influencing flood and drought impacts, the realisation of risks depends on several factors. Floods include river floods, flash floods, urban floods and sewer floods, and can be caused by intense and/or long-lasting precipitation, snowmelt, dam break, or reduced conveyance due to ice jams or landslides. Floods depend on precipitation intensity, volume, timing, antecedent conditions of rivers and their drainage basins (e.g., presence of snow and ice, soil character, wetness, urbanisation, and existence of dikes, dams, or reservoirs). Human encroachment into flood plains and lack of flood response plans increase the damage potential.” [AR4]


 
Adaptation (dealing with the direct effects of climate changes) and mitigation (reducing emissions to prevent further changes) are both essential to climate policy. Adaptation is unavoidable, as even the most aggressive pollution reduction still won’t eliminate some additional warming. Mitigation remains essential, as without mitigation impacts will grow larger than our capacity to adapt.

AR5 Quote: “Even the most stringent mitigation efforts cannot avoid further impacts of climate change in the next few decades, which makes adaptation unavoidable. Without mitigation, a magnitude of climate change is likely to be reached that makes adaptation impossible for some natural systems; while for most human systems it would involve very high social and economic costs.” [WGII Chapter 1, p. 14]

AR4 Quote: “The benefits of mitigation carried out today will be evidenced in several decades because of the long residence time of greenhouse gases in the atmosphere (ancillary benefits such as reduced air pollution are possible in the near term), whereas many adaptation measures would be effective immediately and yield benefits by reducing vulnerability to climate variability. As climate change continues, the benefits of adaptation (i.e., avoided damage) will increase over time. Thus there is a delay between incurring the costs of mitigation and realising its benefits from smaller climate change, while the time span between expenditures and returns of adaptation is usually much shorter.”  [AR4]


 
Climate change will negatively impact many human systems. High agreement between varying studies and sources of evidence exists for significant impacts on transportation infrastructure, the health sector, and insurance systems. Moderate agreement exists for impacts on many more systems.

AR5 Quotes: Climate change may negatively affect transport infrastructure (high agreement, limited evidence). Transport infrastructure malfunctions if the weather is outside the design range, which would happen more frequently as the climate continues to change. All infrastructure is vulnerable to freeze-thaw cycles. Paved roads are particularly vulnerable to temperature extremes, unpaved roads to precipitation extremes. Transport infrastructure on ice or permafrost is especially vulnerable.” [WGII Chapter 10 p. 4]

“Climate change will affect the health sector (high agreement, medium evidence) through increases in the frequency, intensity, and extent of extreme weather events as well as increasing demands for health care services and facilities, including public health programs, disease prevention activities, health care personnel, infrastructure, and supplies related to treatment of infectious deceases and temperature related events.” [WGII Chapter 10 p. 4]

“Climate change will affect insurance systems (high agreement, robust evidence). More frequent and/or intensive weather disasters as projected for some regions/hazards will increase losses and loss variability in various regions and challenge insurance systems to offer affordable coverage while raising more risk-based capital, particularly in low- and middle-income countries. Economic-vulnerability reduction through insurance has proven effective. Large-scale public-private risk prevention initiatives and government insurance of the non-diversifiable portion of risk offer example mechanisms for adaptation. Commercial reinsurance and risk-linked securitization markets also have a role in ensuring financially resilient insurance and risk transfer systems.” [WGII Chapter 10 p. 4]