Basic physical theory and climate models indicate we should expect a suite of physical effects will advance with continued global warming. Temperature increases will likely be larger over land and in polar regions and smaller over the oceans. Weather and natural variability will continue to modulate local and global temperatures, but as average temperatures increase extremes and records will favor hot spells over cold snaps. Warming will be accompanied by increasing amounts of total global precipitation and the amount of precipitation that falls in individual events, because warmer air holds more water vapor. Despite projections of globally increasing rainfall, some areas most likely face a higher tendency toward dry and hot conditions, or drought, as the atmospheric circulation changes. Continued heating of the oceans and melting ice from glaciers will contribute to sea level rise that will likely go on for centuries.
Coincident with the physical effects of climate change, the geochemical impacts of higher CO2 levels will otherwise affect environmental conditions in the oceans and land. As the oceans absorb excess CO2 from the atmosphere, ocean pH will decrease, known as ocean acidification. Higher CO2 concentrations in the atmosphere will make more available for photosynthesis and could act as a fertilizer and aid plant growth and agriculture in some regions.
The risks of climate change come from the effects of climate change clashing with the natural world or human infrastructure and lifestyles. Since climate change implies changes to the background conditions against which society and ecosystems do their work, understanding the potential for positive and negative impacts throughout the planet is challenging. Formal efforts to understand local impacts of climate change have been performed for the world at different levels of global warming, the United States[1], and vulnerable parts of the world for 4 °C of global warming.
Risk assessments of climate change are complicated and relatively uncertain. As we have seen, projections of the future are not certain even for global temperatures. We don’t know with much confidence how regional conditions or extreme weather events will change and likewise how human and natural systems will respond or adapt. The expected pace of 21st century climate change has almost no geological analog. The capacity of society and ecosystems to adapt or adjust to climate change at the rate and size human activities are inciting is untested.
In its fifth assessment report, the IPCC compiled a summary of different studies into the local, regional, and global effects of climate change, the vulnerability of different regions and sectors to change, and the capacity for different systems to adapt to change. Combining those different concepts, the IPCC rated how climate change created additional risks across five different areas.
Risks of global temperature increases to different areas of concern. Temperature increase over preindustrial (right) and near present (left) are shown by the thermometer graphics. Each bar represents a different area of concern. The different levels of additional risk are shown as transitioning colors, going from undetectable (white) to very high (purple). Source: IPCC AR5 Working Group 2, Chapter 19 Emergent Risks and Key Vulnerabilities.
The IPCC’s ratings are subjective, but they describe the character of additional risk associated with each level of warming. At a medium level of additional risk, the climate change signature will begin to be detectable in specific events or regions or sporadically harmful. As risks move from high to very high, the effects of climate change become more pervasive, harmful, or permanent.
The temperature increase at which each area of concern transitions to higher levels of additional risk varies, but there is general agreement that risks increase with temperature. At this time, we are only edging into moderate levels of additional risk to unique and threatened systems and extreme weather. However, once we surpass 2 °C of warming, the additional risk in each category will range from medium to high. At 4 °C of warming over preindustrial levels, the additional risk is high to very high across all sectors.