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. p.xiii
Indeed, present emission trends put the world plausibly on a path toward 4°C warming within the century. p.xiii
• Sea-level rise is likely to be 15 to 20 percent 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. p.xiii
A world in which warming reaches 4°C above preindustrial levels (hereafter referred to as a 4°C world), would be one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on human systems, ecosystems, and associated services. p.xiv
OBSERVED IMPACTS AND CHANGES TO THE CLIMATE SYSTEM (worsening since the 2007 IPCC predictions)
• The concentration of the main greenhouse gas, carbon dioxide (CO2), has continued to increase from its preindustrial concentration of approximately 278 parts per million (ppm) to over 391 ppm in September 2012, with the rate of rise now at 1.8 ppm per year.
• The present CO2 concentration is higher than paleoclimatic and geologic evidence indicates has occurred at any time in the last 15 million years.
• Emissions of CO2 are, at present, about 35,000 million metric tons per year (including land-use change) and, absent further policies, are projected to rise to 41,000 million metric tons of CO2 per year in 2020.
• Global mean temperature has continued to increase and is now about 0.8°C above preindustrial levels. p.xiv
The average increase in sea levels around the world over the 20th century has been about 15 to 20 centimeters. Over the last decade the average rate of sea-level rise has increased to about 3.2 cm per decade. Should this rate remain unchanged, this would mean over 30 cm of additional sea-level rise in the 21st century.
Overall, the rate of loss of ice has more than tripled since the 1993–2003 period as reported in the IPCC AR4, reaching 1.3 cm per decade over 2004–08; the 2009 loss rate is equivalent to about 1.7 cm per decade. If ice sheet loss continues at these rates, without acceleration, the increase in global average sea level due to this source would be about 15 cm by the end of the 21st century. p.xiv
The last decade has seen an exceptional number of extreme heat waves around the world with consequential severe impacts. Human-induced climate change since the 1960s has increased the frequency and intensity of heat waves and thus also likely exacerbated their societal impacts. In some climatic regions, extreme precipitation and drought have increased in intensity and/or frequency with a likely human influence. p.xiv
In the absence of climate change, extreme heat waves in Europe, Russia, and the United States, for example, would be expected to occur only once every several hundred years. Observations indicate a tenfold increase in the surface area of the planet experiencing extreme heat since the 1950s. p.xiv
Negative effects of higher temperatures have been observed on agricultural production, with recent studies indicating that since the 1980s global maize and wheat production may have been reduced significantly compared to a case without climate change. p.xiv
An MIT study used historical fluctuations in temperature within countries to identify its effects on aggregate economic outcomes. It reported that higher temperatures substantially reduce economic growth in poor countries and have wide-ranging effects, reducing agricultural output, industrial output, and political stability. These findings inform debates over the climate’s role in economic development and suggest the possibility of substantial negative impacts of higher temperatures on poor countries. p.xv
(Dell, Melissa, Benjamin F. Jones, and Benjamin A. Olken. 2012. “Temperature Shocks and Economic Growth: Evidence from the Last Half Century.” American Economic Journal: Macroeconomics, 4(3): 66–95.)
PROJECTED CLIMATE CHANGE IMPACTS IN A 4° WORLD
Projections for a 4°C world show a dramatic increase in the intensity and frequency of high-temperature extremes. Recent extreme heat waves such as in Russia in 2010 are likely to become the new normal summer in a 4°C world p.xv
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. p.xv
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. p.xv
The observed and projected rates of change in ocean acidity over the next century appear to be unparalleled in Earth’s history. p.xv
Coral reefs in particular are acutely sensitive to changes in water temperatures, ocean pH, and intensity and frequency of tropical cyclones p.xv
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. p.xv
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. Sea-level rise would likely be limited to below 2 meters only if warming were kept to well below 1.5°C. p.xv
Sea-level rise will vary regionally: for a number of geophysically determined reasons, it is projected to be up to 20 percent 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 p.xv
Sea-level rise impacts are projected to be asymmetrical even within regions and countries. p.xvi
Highly vulnerable cities are to be found in Mozambique, Madagascar, Mexico, Venezuela, India, Bangladesh, Indonesia, the Philippines, and Vietnam. p.xvi
RISKS TO HUMAN SUPPORT SYSTEMS : FOOD, WATER, ECOSYSTEMS, AND HUMAN HEALTH
The risk for disruptions to ecosystems as a result of ecosystem shifts, wildfires, ecosystem transformation, and forest dieback would be significantly higher for 4°C warming as compared to reduced amounts. Increasing vulnerability to heat and drought stress will likely lead to increased mortality and species extinction. p.xvi
In Amazonia, forest fires could as much as double by 2050 with warming of approximately 1.5°C to 2°C above preindustrial levels. Changes would be expected to be even more severe in a 4°C world. pxvi
Recent research suggests that large-scale loss of biodiversity is likely to occur in a 4°C world, with climate change and high CO2 concentration driving a transition of the Earth´s ecosystems into a state unknown
in human experience. p.xvi
Maintaining adequate food and agricultural output in the face of increasing population and rising levels of income will be a challenge irrespective of human-induced climate change. The IPCC AR4 projected that global food production would increase for local average temperature rise in the range of 1°C to 3°C, but may decrease beyond these temperatures. p.xvi
New results published since 2007, however, are much less optimistic. These results suggest instead a rapidly rising risk of crop yield reductions as the world warms. Large negative effects have been observed at high and extreme temperatures in several regions including India, Africa, the United States, and Australia. p.xvi
Sea-level rise would likely impact many mid-latitude coastal areas and increase seawater penetration into coastal aquifers used for irrigation of coastal plains. p.xvii
Recent projections suggest that the poor are especially sensitive to increases in drought intensity in a 4°C world, especially across Africa, South Asia, and other regions. p.xvii
Whilst economic growth is projected to significantly reduce childhood stunting, climate change is projected to reverse these gains in a number of regions: substantial increases in stunting due to malnutrition are projected to occur with warming of 2°C to 2.5°C, especially in Sub-Saharan Africa and South Asia, and this is likely to get worse at 4°C. p.xvii
RISKS OF DISRUPTIONS AND DISPLACEMENTS IN A 4°C WORLD
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. p.xvii
With pressures increasing as warming progresses toward 4°C and combining with nonclimate–related social, economic, and population stresses, the risk of crossing critical social system thresholds will grow. At such thresholds existing institutions that would have supported adaptation actions would likely become much less effective or even collapse. p.xviii
stresses on human health, such as heat waves, malnutrition, and decreasing quality of drinking water due to seawater intrusion, have the potential to overburden health-care systems to a point where adaptation is
no longer possible, and dislocation is forced. p.xviii
A 4°C world is likely to be one in which communities, cities and countries would experience severe disruptions, damage, and dislocation, with many of these risks spread unequally. It is likely that the poor will suffer most and the global community could become more fractured, and unequal than today. p.xviii