Without further commitments and action to reduce greenhouse gas emissions, the world is likely to warm by more than 3 °C above the preindustrial climate. Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4 °C by 2100. If they are not met, a warming of 4 °C could occur as early as the 2060s.

Such a warming level and associated sea-level rise of 0.5 to 1 meter, or more, by 2100 would not be the end point: a further warming to levels over 6 °C, with several meters of sea-level rise, would likely occur over the following centuries.

Thus, while the global community has committed itself to holding warming below 2 °C to prevent “dangerous” climate change, and Small Island Developing states (SIDS) and Least Developed Countries (LDCs) have identified global warming of 1.5 °C as warming above which there would be serious threats to their own development and, in some cases, survival, the sum total of current policies—in place and pledged—will very likely lead to warming far in excess of these levels. Indeed, present emission trends put the world plausibly on a path toward 4°C warming within the century.

—“Turn Down the Heat”

The report authors acknowledge that uncertainties remain in projecting the extent of both climate change and its impacts. For their report, they took a risk-based approach in which risk is defined as impact multiplied by probability—e.g., an event with low probability can still pose a high risk if it implies serious consequences.

The distribution of impacts is likely to be inherently unequal and tilted against many of the world’s poorest regions, which have the least economic, institutional, scientific, and technical capacity to cope and adapt, the report finds. For example:

• Even though absolute warming will be largest in high latitudes, the warming that will occur in the tropics is larger when compared to the historical range of temperature and extremes to which human and natural ecosystems have adapted and coped. The projected emergence of unprecedented high-temperature extremes in the tropics will consequently lead to significantly larger impacts on agriculture and ecosystems.

• Sea-level rise is likely to be 15–20% larger in the tropics than the global mean.

• Increases in tropical cyclone intensity are likely to be felt disproportionately in low-latitude regions.

• Increasing aridity and drought are likely to increase substantially in many developing country regions located in tropical and subtropical areas.

Key findings on the range of the direct and indirect climatic consequences under the current global path for greenhouse gas emissions include:

• Projected climate change impacts in a 4°C world. The largest warming will occur over land and range from 4 °C to 10 °C. Increases of 6 °C or more in average monthly summer temperatures would be expected in large regions of the world, including the Mediterranean, North Africa, the Middle East, and the contiguous United States.

  Projections for a 4°C world show a dramatic increase in the intensity and frequency of high-temperature extremes. Tropical South America, central Africa, and all tropical islands in the Pacific are likely to regularly experience heat waves of unprecedented magnitude and duration. In this new high-temperature climate regime, the coolest months are likely to be substantially warmer than the warmest months at the end of the 20_th century, according to the report.

  In regions such as the Mediterranean, North Africa, the Middle East, and the Tibetan plateau, almost all summer months are likely to be warmer than the most extreme heat waves presently experienced. For example, the warmest July in the Mediterranean region could be 9°C warmer than today’s warmest July.

  The impacts of the extreme heat waves projected for a 4°C world have not been evaluated, but they could be expected to vastly exceed the consequences experienced to date and potentially exceed the adaptive capacities of many societies and natural systems.

• Rising CO₂ concentration and ocean acidification. Apart from a warming of the climate system, one of the most serious consequences of rising carbon dioxide concentration in the atmosphere occurs when it dissolves in the ocean and results in acidification. A warming of 4 °C or more by 2100 would correspond to a CO₂ concentration above 800 ppm and an increase of about 150% in acidity of the ocean. The observed and projected rates of change in ocean acidity over the next century appear to be unparalleled in Earth’s history. Evidence is already emerging of the adverse consequences of acidification for marine organisms and ecosystems, combined with the effects of warming, overfishing, and habitat destruction.

  Coral reefs in particular are acutely sensitive to changes in water temperatures, ocean pH, and intensity and frequency of tropical cyclones. Reefs provide protection against coastal floods, storm surges, and wave damage as well as nursery grounds and habitat for many fish species. Coral reef growth may stop as CO₂ concentration approaches 450 ppm over the coming decades (corresponding to a warming of about 1.4°C in the 2030s), the report notes.

  By the time the concentration reaches around 550 ppm (corresponding to a warming of about 2.4 °C in the 2060s), it is likely that coral reefs in many areas would start to dissolve. The combination of thermally induced bleaching events, ocean acidification, and sea-level rise threatens large fractions of coral reefs even at 1.5 °C global warming. The regional extinction of entire coral reef eco- systems, which could occur well before 4 °C is reached, would have profound consequences for their dependent species and for the people who depend on them for food, income, tourism, and shoreline protection.

• Rising sea levels, coastal inundation and loss. Warming of 4 °C will likely lead to a sea-level rise of 0.5 to 1 meter, and possibly more, by 2100, with several meters more to be realized in the coming centuries. Limiting warming to 2 °C would likely reduce sea-level rise by about 20 cm by 2100 compared to a 4 °C world. However, even if global warming is limited to 2 °C, global mean sea level could continue to rise, with some estimates ranging between 1.5 and 4 meters above present-day levels by the year 2300, the report highlights. Sea-level rise by then would likely be limited to below 2 meters only if warming were kept to well below 1.5 °C.

  Sea-level rise will vary regionally. For a number of geophysically determined reasons, it is projected to be up to 20% higher in the tropics and below average at higher latitudes. In particular, the melting of the ice sheets will reduce the gravitational pull on the ocean toward the ice sheets and, as a consequence, ocean water will tend to gravitate toward the Equator. Changes in wind and ocean currents due to global warming and other factors will also affect regional sea-level rise, as will patterns of ocean heat uptake and warming.

• Risks to human support systems: food, water, ecosystems, and human health. Although impact projections for a 4°C world are still preliminary and it is often difficult to make comparisons across individual assessments, this report identifies a number of severe risks for vital human support systems. With extremes of temperature, heat waves, rainfall, and drought are projected to increase with warming; risks will be much higher in a 4 °C world compared to a 2 °C world.

  In a world rapidly warming toward 4 °C, the most adverse impacts on water availability are likely to occur in association with growing water demand as the world population increases. Some estimates indicate that a 4 °C warming would significantly exacerbate existing water scarcity in many regions, particularly northern and eastern Africa, the Middle East, and South Asia, while additional countries in Africa would be newly confronted with water scarcity on a national scale due to population growth.

• Risks of disruptions and displacements in a 4 °C world. Economic growth and population increases over the 21^st century will likely add to human welfare and increase adaptive capacity in many, if not most, regions. At the same time, however, there will also be increasing stresses and demands on a planetary ecosystem already approaching critical limits and boundaries. The resilience of many natural and managed ecosystems is likely to be undermined by these pressures and the projected consequences of climate change, the report suggests.

  The projected impacts on water availability, ecosystems, agriculture, and human health could lead to large-scale displacement of populations and have adverse consequences for human security and economic and trade systems. The full scope of damages in a 4°C world has not been assessed to date.

Large-scale and disruptive changes in the Earth system are generally not included in modeling exercises, and rarely in impact assessments. As global warming approaches and exceeds 2 °C, the risk of crossing thresholds of nonlinear tipping elements in the Earth system, with abrupt climate change impacts and unprecedented high-temperature climate regimes, increases.

—“Turn Down the Heat”

The Earth system’s responses to climate change appear to be non-linear. If we venture far beyond the 2 degrees guardrail, towards the 4 degrees line, the risk of crossing tipping points rises sharply. The only way to avoid this is to break the business-as-usual pattern of production and consumption.

—PIK Director, John Schellnhuber

The world must tackle the problem of climate change more aggressively. Greater adaptation and mitigation efforts are essential and solutions exist. We need a global response equal to the scale of the climate problem, a response that puts us on a new path of climate smart development and shared prosperity. But time is very short.

—Jim Yong Kim, President, World Bank Group

The World Bank suggests that initiatives could include: putting the more than US$1 trillion of fossil fuel and other subsidies to better use; introducing natural capital accounting into national accounts; expanding both public and private expenditures on green infrastructure able to withstand extreme weather and urban public transport systems designed to minimize carbon emission and maximize access to jobs and services; supporting carbon pricing and international and national emissions trading schemes; and increasing energy efficiency-- especially in buildings-- and the share of renewable power produced.

The World Bank is supporting 130 countries in taking action on climate change.The World Bank doubled lending for climate change adaptation last year and plans to step up efforts to support countries’ initiatives to mitigate carbon emissions and promote inclusive green growth and climate-smart development. Among other measures, the Bank administers the $7.2 billion Climate Investment Funds now operating in 48 countries and leveraging an additional $43 billion in clean investment and climate resilience.

Resources

• Turn Down the Heat