1.4: Impacts of Climate Change

Climate change affects all aspects of life on the planet, including ecosystems, social systems, economics, public health, urban systems, and rural systems. The observed warming of 1°C is already having an impact on these systems. With unchecked emissions, warming could reach unmanageable levels this century. It may be better to call it climate disruption rather than climate change.

As shown in Figure 1.4.1, the Earth’s climate has varied significantly over the last 1,000 years. Global records for this period are not available, but proxy records such as tree rings and pollen suggest that the northern half of the Northern Hemisphere experienced significant warming (0.5°C) during the Medieval Warm Period from AD 950 to 1250. While Europe enjoyed the warmth and Vikings traveled westward to found settlements in Greenland, other regions, including the American Southwest, suffered from megadroughts and heat waves. The unlucky regions included North America, Central and South America, and northern China. The legendary city and massive temple complex of Angkor Wat in Cambodia were abandoned largely because of decades-long megadroughts interrupted by occasional episodes of intense rainfall and flooding. The Medieval Warm Period was followed by the Little Ice Age from about the mid-1600s to the mid-1800s, which saw widespread cooling over the North Atlantic and Europe, with global temperatures on the order of 0.5°C cooler than in the mid-twentieth century. The Thames River in London froze over multiple times during this period.

These climate events serve to illustrate the strong vulnerability of civilizations to climate change. However, the large climate changes experienced during the past 1,000 years cannot be assumed to be a reliable guide for expected climate changes in the coming decades, in part because the Medieval Warm Period was neither global nor widespread, even over the Northern Hemisphere. We will begin with the documented impacts of twentieth-century warming on a global scale. As we will see, temperature changes during the twentieth and twenty-first centuries have been larger than those of either the Medieval Warm Period or the Little Ice Age, with significant climate impacts. After that, we will look at the projected impacts of continued warming during the twenty-first century.

Current impacts: twentieth and early twenty-first centuries

About two-thirds of the 1°C warming recorded since the beginning of the twentieth century has occurred in the past four decades, starting around 1980. As of this writing (2019), 2009 to 2018 has been the hottest 10-year period on record.

As previously discussed, a warmer atmosphere holds more water vapor, so as the planet warms, it becomes more humid. Warmer temperatures also increase the overall cycle of water evaporation and precipitation, making drier regions even drier and wetter regions wetter. Dry areas worldwide have increased from about 15% of the Earth’s land surface during the mid-twentieth century to about 30% by the first decade of the twenty-first century.

The last two decades have also witnessed record increases in extreme weather events. The incidence of very strong hurricanes (category 4 and 5) has increased at the rate of about 25% per degree of global averaged warming. The number of disastrous floods has increased from less than 50 per year during the mid-twentieth century to more than 150 per year during the first decade of the twenty-first century.

How do we know the increase in extreme weather is due to anthropogenic global warming? The science of attributing individual extreme events to climate change has improved significantly. Multiple factors are involved in any extreme event, so it’s not possible to say that a specific weather event was “caused” by global warming, but we can determine how much more likely that kind of event is, given the increased temperatures. For example, the record Russian heat wave of 2010, which claimed 15,000 lives, as well as many of the major storms and droughts witnessed in 2016, have all been statistically attributed to global warming with about 80% certainty. That is, there is a four in five (80%) chance that the Russian heat wave would not have occurred in the absence of human-induced climate change. It’s estimated that widespread warming and rising humidity increased the probability of extreme weather, particularly heat waves, by a factor of 10 or more from 2011 to 2015. An analysis of 170 reports on 190 extreme weather events from 2004 to mid-2018 suggests that about two-thirds of these extreme weather events were made more likely, or more severe, by anthropogenic climate change.

The impacts of global warming can also be seen in its effects on ice and sea levels around the planet. Since 1980, the summer extent of Arctic sea ice has decreased by as much as 10% to 15%. Glaciers are melting worldwide. Major ice sheets, particularly in Greenland and West Antarctica, are losing mass at a significant rate. Sea levels are rising at a rate of about 3 millimeters per year because of the melting of glaciers and ice sheets and the expansion of seawater as the ocean warms. The ocean is also becoming more acidic because of absorption of CO₂, which produces carbonic acid. The changes described above have had significant impacts on natural ecosystems as well as human society and human health. A few of the observed impacts are detailed in Box 1.4.1.

Box 1.4.1 Observed Impacts of Global Warming (Late Twentieth and Early Twenty-First Centuries)

Impacts on ecosystems

• As we saw earlier in this chapter, trees and other plants absorb and store carbon dioxide from the atmosphere. Prior to the twenty-first century, tropical forests acted as a net absorber (sink) of carbon dioxide. For example, a young growing tree would absorb carbon in carbon dioxide, while a dying tree would release that carbon back to the air. However, during the first decade of the twenty-first century, tropical forests became a net source of CO₂ because of degradation from drought and warming.
• Corals get most of their energy from single-cell, photosynthetic organisms that live in their tissues. However, if water temperatures are too warm, the corals expel these photosynthesizing organisms and are left as white skeletons. This is called coral bleaching. If warm conditions persist for weeks or months, the coral may die. Coral bleaching due to warming is happening in most coral reefs; the most severe global bleaching event in recorded history occurred from 2015 to 2017. During this period, it is estimated, as much as half of the coral in Australia’s Great Barrier Reef was killed.

Impacts on human societies and human health

• Warming and droughts have increased water demand over 86% of cropping area by about 2.3% to 3.6% per decade since 1981, contributing to significant reductions in wheat yield and increase in plant diseases.
• Adverse health impacts of climate change, such as heat stress, have been documented extensively. The Lancet Commissions, which consists of international experts in public health, air pollution, and climate change, concluded in 2015 that the “effects of climate change are being felt today, and future projections represent an unacceptably high and potentially catastrophic risk to human health.”
• Threats to health, both physical and mental, also arise from decreases in food security and water availability. These threats include increases in waterborne diseases such as childhood gastrointestinal diseases caused by floods. Due to worldwide increases in temperature and humidity, insect-borne diseases, such as malaria, dengue fever, Lyme disease, and chikungunya, are migrating outside the tropics and to higher altitudes.
• The number of people displaced because of weather extremes has increased to 21 million people.

Climate change to climate disruption

For the first time, the statistical barrier against identification of climate change as causal factor for extreme weather events was overcome. The scientifically cautious American Meteorological Association (AMS) issued the remarkable statement:

For years scientists have known humans are changing the risk of some extremes. But finding multiple extreme events that weren’t even possible without human influence makes clear that we’re experiencing new weather, because we’ve made a new climate.

The United Nations Office for Disaster Risk Reduction estimates that from 1995 to 2015, weather-related disasters have claimed 606,000 lives; furthermore 4.1 billion people have been injured, made homeless, or required emergency assistance. In addition, the UN agency estimates the number of disasters during the latter half of the 20-year period was double that of the first 10-year period. Climate change is thus bringing new weather extremes and fatal catastrophes—meaning that climate change is better termed climate disruption. Unchecked climate change is likely to become unmanageable. That could happen in a matter of few to several decades as discussed next.

The next three decades: impacts of 2°C warming

As of 2010, we have already emitted 2 trillion tons of carbon dioxide. As discussed earlier in this chapter, nearly half of that amount is still in the atmosphere—990 billion tons of CO₂, trapping 860 terawatts of heat energy. Since 2010, we have added another 200 billion tons, bringing the total to 2.2 trillion tons as of 2018. Even if we were to stop emissions immediately, the Earth would warm by another 0.5°C by 2030 to compensate for the heat energy trapped by the already emitted CO₂, along with non-CO₂ pollutants. Emissions to date have already committed us to this 0.5°C rise in temperature, which would bring the total warming since 1850 to 1.5°C. For comparison, even the 1°C of warming experienced during the Eemian interglacial 130,000 years ago was sufficient to increase sea level by 6 to 9 meters.

At current emission growth levels, under a “business as usual” scenario, we will add another trillion tons of carbon dioxide to the atmosphere within the next 15 years, by about 2030. This additional carbon dioxide is likely (with a probability of at least 50%) to mean that total warming will exceed 2°C before 2050. At that point, the decadal rate of climate change will be three times faster than the pace experienced until now. Most climate scientists and ecologists concur that 1.5°C to 2°C represents the warming threshold for dangerous climate impacts.

The impacts of 2°C warming would be quite severe. Rising temperatures will result in an increase in the frequency and duration of severe heat waves. It’s estimated that with 2°C warming, well over 3 billion people—about 40% of the human population by 2050—would experience summer mean temperatures hotter than the current record hottest summers in one out of every two years. Moreover, about 1.8 billion people would be exposed to lethal heat for more than 20 days a year. Increasing temperatures will also lead to more droughts and wildfires, as well as increases in severe storms and flooding.

One impact with truly global consequences is sea level rise. Even in the unlikely event that warming is stabilized at 1.5°C, sea level rise will continue for centuries because of ongoing melting of the Greenland and West Antarctic ice sheets. Studies of data for the past million years suggest that a 1°C warming (equivalent to the Eemian warming) is sufficient to lead to an eventual sea level rise of 6 to 9 meters over several centuries, and a 2°C warming could lead to a rise of 6 to 13 meters. Since more than 75% of the population will be living in coastal cities by the end of this century, sea level rise of such magnitudes has enormous negative implications for displacement and mass migration of people, disruption of social systems, and exacerbation or creation of international conflicts.

Box 1.4.2 provides some additional examples of the impacts of 2°C warming.

Box 1.4.2 Impacts of 2°C Warming

• Highly populated regions, such as the eastern and western United States, Middle East, South Asia, and China, could experience heat waves worse than the most severe Russian heat wave of 2010, when temperatures reached 55°C (131°F).
• About 600 million additional people will be exposed to dengue, chikungunya, and many other viruses because of the expanded range of disease-carrying mosquitoes.
• Moderate to severe widespread droughts and fires will occur worldwide. Both rural and urban populations will be affected by air pollution, loss of property, and land degradation that reduces food production and contributes to volatile food prices.
• Floods and storms will become more frequent and/or intense. Scientists are still debating whether the frequency of hurricanes will increase, but the storms that do occur are expected to be stronger, meaning an increase in the strongest (category 4 and 5) hurricanes.
• The climate could reach a tipping point when forest recovery time increases to more than 55 months and the intervals between droughts decrease to less than 55 months. If that happens, forests may not recover from droughts and fires.

The late twenty-first century: warming of 4°C or more

By the end of this century, a business-as-usual path with unchecked emissions will lead to warming that could exceed 4°C. As we discussed in Section 1.3, projections by models give a range of possible future temperatures because of differing model treatments of climate feedbacks that can either amplify the warming or moderate it. These feedbacks, as discussed earlier, include increasing water vapor in the atmosphere and the melting of Arctic sea ice, replacing the reflective ice surface with open ocean waters that absorb additional solar radiation and amplify warming. We also saw that one of the largest sources for the temperature range projected by climate models is differing projections of how the amount and distribution of clouds will change in a warming world. Because of this, scientists express projections of future temperature in terms of a range of probabilities rather than a single temperature.

The curves in Box 1.4.3 show the probability of various levels of warming for different scenarios of emissions growth. The three curves in red and brown to the right side of the curve are for scenarios in which emissions growth is essentially unchecked.

Box 1.4.3 Dangerous to Existential Risk: Categories of Projected 2100 Warming

The green curve on the left labeled “10 Solutions” represents a scenario in which emissions are curtailed or phased out completely, employing the ten solutions described in Chapter 4. The other three curves in red and brown represent scenarios with unchecked emissions.

For the red and brown curves, BL = baseline, meaning no significant mitigation efforts. CI = carbon intensity, referring to the amount of carbon dioxide emitted per unit of the global economy. Because of shifts in the economy and increasing costs of fossil fuels, it’s expected that the carbon intensity of the economy will decrease even without significant mitigation efforts. However, because the world economy will continue to grow, actual carbon emissions are expected to increase. For example, if the carbon intensity of the world economy were to decrease by half while the economy grew to four times its present size, total emissions would double.

There are specific scenarios shown: BL (CI–80%), the lowestemission scenario of the three, in which carbon intensity decreases 80% by 2100; BL (CI–0%) in which carbon intensity decreases 50% by 2100; and BL (CI–50% & C feedback), which is the same as the second scenario except that it also accounts for feedbacks such as a decrease in carbon dioxide uptake by soils as temperatures increase, meaning that more carbon dioxide would stay in the atmosphere.

Source: Ramanathan, V., et al. Well Under 2 Degrees Celsius: Fast Action Policies to Protect People and the Planet from Extreme Climate Change, 2017. Image from Figure 1. http://www.igsd.org/wp-content/uploa...eport-2017.pdf.

Box 1.4.4 Impacts of 4°C Warming or Greater

• Warming of 4°C would likely expose over 70% of the population (this would be about 7.5 billion people by 2100) to lethal heat waves. More than 2.5 billion people could be exposed to diseases carried by mosquitoes and other pests.
• Warming of 4°C would likely expose about 20% of natural species to extinction. This is in addition to the roughly 50% or more of species that will be exposed to extinction through habitat destruction by the 11 billion humans populating the planet by 2100. An extinction rate of 70% or more is considered to be a mass extinction similar to what happened during the Cretaceous period when dinosaurs disappeared from the planet.
• Over several centuries, warming greater than 5°C could result in an ice-free Earth, with a rise in sea level of more than 90 meters. Widespread droughts are likely the most serious outcome, threatening food and water for most of the 11 billion people expected to be on the planet by 2100 (Figure 1.4.2).
• These impacts will be in addition to worsening droughts, floods, fires, storms, hurricanes, and dying forests. Widespread droughts are likely the most serious outcome, threatening food and water security for most of the 11 billion people expected to be on the planet by 2100.
• These weather extremes, sea level rise, and the spread of vector-borne viral diseases will likely lead to the mass migration of millions of human beings.

Figure 1.4.2 Soil moisture conditions in 2080–2099 with unchecked emissions as simulated by the Princeton University climate model. Adapted from Cook et al. 2015.

The key point is that continued growth in emissions would result in temperatures that expose human society and natural ecosystems to very severe threats. In these scenarios, the likely warming by 2100 ranges from less than 3°C to more than 7°C. There is less than 10% probability that the warming will be less than 3°C, and less than 10% probability that it will be greater than 7°C.

Warming in excess of 4°C would produce catastrophic changes, while warming of 5°C or more would have impacts so severe that it could pose an existential threat to society, as illustrated by the findings in Box 1.4.4.

One type of high-impact consequence that is of major concern is the possibility of runaway feedbacks. For example, large temperature increases could result in methane release by warming permafrost and wetlands and the disappearance of sea ice and glaciers. This could start a feedback loop in which higher temperatures cause more methane to be released, in turn causing further warming. Such a feedback loop would be outside human control and could undo much of the benefit of any reductions in anthropogenic emissions.

Extreme temperature increases of 6°C or greater are an example of low-probability, high-impact events. While the chance that these events will occur may be comparatively low, their consequences would be so severe that significant efforts to avoid them are warranted. One way to think about this is to consider the question, Would you get on a plane if you knew there was a 10% chance it would crash? While there is a high probability (90%) that you would survive the flight, the severe consequences of the “low-probability” crash might make you rethink your plans.

However, the green curve in Box 1.4.3, labeled “10 Solutions,” represents the warming probability if the ten climate solutions presented in Chapter 4 are implemented. Note that this curve gives a high probability of remaining below the 2°C threshold for dangerous climate change. The green curve shows us there is real hope that if we act now, we will be able to avoid the most serious negative consequences of climate change.