Climate stability

Lane Kenworthy, The Good Society
May 2019

Are we causing the planet to warm? What are the potential consequences? What, if anything, should we do about it?


The hypothesis is relatively straightforward: In various ways, but especially by burning fossil fuels, we’ve dramatically increased greenhouse gas emissions. If the amount of greenhouse gases emitted from earth is larger than natural processes can remove, it will trap more infrared radiation (sunlight that bounces off the earth) in the atmosphere, leading to a rise in temperature. Also, warm air holds more water vapor than cold air, and water vapor causes additional trapping of heat. The planet should therefore be warming.

What does the evidence say?

Begin with the hypothesized cause. As figure 1 shows, emissions of carbon dioxide have increased steadily since 1750, when the industrial revolution began, and the rise has been especially pronounced since 1950.

Figure 1. Carbon dioxide emissions
“g” = gigatonnes. Data source: Carbon Dioxide Information Analysis Center,

As carbon dioxide emissions have increased, the amount of carbon dioxide in the atmosphere has increased. We have direct measurement of CO2 atmospheric concentration only since 1959, but data from ice core drilling in Antarctica suggest that in the 800,000 years prior to 1750, it never exceeded 300 ppm (parts per million). Between 1750 and 1959 it rose from 280 to 316. As figure 2 indicates, between 1959 and 2018 it increased from 316 to 409.1

Figure 2. Carbon dioxide concentration in the atmosphere
Parts per million. 280 is the estimated level in 1750. Data source: Earth System Research Laboratory, National Oceanic and Atmospheric Administration, US Department of Commerce,

Carbon dioxide isn’t the only problematic greenhouse gas. Methane and nitrous oxide play a role too. Here a major contributor is the raising of animals for food, which accounts for approximately 15% of all greenhouse gas emissions.2 That’s roughly the same as all cars, trucks, airplanes, and ships combined.3 Figure 3 shows that methane concentration in the atmosphere has increased sharply since scientists began measuring it in the mid-1980s. The concentration of methane is much lower than of carbon dioxide, but methane’s potency as a greenhouse gas is about 30 times greater.4

Figure 3. Methane concentration in the atmosphere
Parts per billion. 680 is the estimated level in 1750. Data source: Earth System Research Laboratory, National Oceanic and Atmospheric Administration, US Department of Commerce,

Next, the hypothesized effect. Has the planet’s temperature increased? Yes, it has. Figure 4 shows direct measurements beginning in 1880. Average temperature has risen sharply, particularly since 1950. Figure 5 shows indirect and direct measurements going back to 700. It suggests a similar conclusion.

Figure 4. Earth’s average temperature
Difference from the 1901-2000 average. Degrees Celcius. Land and ocean. The line is a loess curve. Data source: National Centers for Environmental Information, National Oceanic and Atmospheric Administration,

Figure 5. Earth’s average temperature
Source: New York Times, using data from the Intergovernmental Panel on Climate Change.

Because the historical temperature records are incomplete, scientists introduce corrections. Are these corrections biased? This was the concern at the heart of the 2009 “climategate” controversy. However, the Berkeley Earth Surface Temperature (BEST) project has gathered together all existing temperature measures, and the data, even with no corrections, show a similar trend.5

Is this a “heat island” effect? In other words, do these measurements show warming simply because a number of the temperature stations are near towns that have been growing? No. We know this for three reasons: this effect is likely to be strongest on still nights, yet trends from data recorded on still nights are similar to those on windy nights; temperature readings from non-heat-island stations show a similar trend to those from heat-island stations; and the temperature of water at the surface of oceans shows a similar trend to that on land.6

Could the observed temperature rise be a result of something else? Probably not. Scientists use computer simulations that allow them to project climate effects in scenarios with little or no human emission of greenhouse gases. According to the simulations, none of those scenarios can produce the temperature patterns we observe. When the simulations add the greenhouse gases that we know humans are generating, they come close to matching the observed temperature patterns, as figure 6 shows.

Figure 6. Predicted temperature and actual temperature
The predicted temperature, shown in red, is based on levels of carbon dioxide in the atmosphere and volcanic activity. The actual temperature, shown in black, is the average land surface temperature for the planet (degrees Celsius), ten-year moving average. Source: Berkeley Earth Surface Temperature (BEST) project, “Summary of Results,” 2013.

Also, as Joseph Romm notes, “if the warming is caused by an increase in greenhouse gases, we expect the lower atmosphere (troposphere) to warm, the upper atmosphere (stratosphere) to cool, and the boundary between them (tropopause) to rise. All of this has been observed. If, for instance, recent warming were due to increases in the intensity of radiation from the sun, then in addition to the troposphere, the stratosphere should be warming, too, which is not happening.”7

The most thorough survey of the evidence is a 2013 report by the Intergovernmental Panel on Climate Change (IPCC). It concluded that “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”8

In sum, while it’s impossible to be fully certain, the computer simulations coupled with the observed evidence yield a conclusion of “very likely” that humans are causing climate change.

How much consensus is there among climate scientists? In 2010, the Proceedings of the National Academy of Sciences reported the findings of a survey of 1,372 climate researchers. It found that 97-98% of those publishing in the field believe humans are causing climate change. And “The relative climate expertise and scientific prominence of the unconvinced researchers … are substantially below that of the convinced researchers.”9 A more recent study also found 97% agreement.10

A 2014 report by the Climate Change Panel of the American Association for the Advancement of Science described the scientific consensus this way: “The science linking human activities to climate change is analogous to the science linking smoking to lung and cardiovascular diseases. Physicians, cardiovascular scientists, public health experts, and others all agree smoking causes cancer. And this consensus among the health community has convinced most Americans that the health risks from smoking are real. A similar consensus now exists among climate scientists, a consensus that maintains climate change is happening, and human activity is the cause.”11


One consequence of rising temperatures will be more hot or very hot days. Figure 7 shows that the number of 100-degree-or-more days per year is projected to increase significantly across the United States. Climate change also will increase the frequency and intensity of storms and of droughts. Extreme rainstorms have become more common in the United States in recent decades, as we see in figure 8. And climate change may have damaging effects on our health via heat waves, urban smog, and lack of access to water.12



Figure 7. Number of days above 100 degrees if greenhouse gas emission trends continue unabated
Fahrenheit. Source: Heidi Cullen, “Think It’s Hot Now? Just Wait,” New York Times, 2016, using projections by the World Climate Research Program.

Figure 8. Extreme rainstorms
Share of US weather stations experiencing an extreme rainstorm in a year (10-year average). The definition of “extreme” varies by region, depending on typical rainfall. Data source: David Leonhardt, “Irma, and the Rise of Extreme Rain,” New York Times, 2017, using data from Kenneth Kunkel, North Carolina Institute for Climate Studies.

The most significant impact of warming could come via melting of the Greenland and West Antarctic ice sheets, each of which has enough ice to raise sea levels by 15 to 20 feet. A significant rise in sea level could threaten hundreds of millions of people who live in coastal areas. Predicting how much the two ice sheets will melt and what impact that will have is extremely difficult. In its 2013 report, the IPCC concluded that melting from the two ice sheets may cause a sea level rise of 1 to 3 feet by the year 2100. Since then, scientists have discovered that ice loss in the Greenland and West Antarctic ice sheets has been faster in recent years than previously thought and that a large glacier in the East Antarctic ice sheet is now vulnerable to melting. According to one knowledgable observer, these recent findings “have led top climatologists to conclude that we are likely headed toward what used to be the high-end of projected global sea-level rise this century (i.e., 4 to 5 feet) and that the worst-case scenarios where humanity fails to take aggressive action to cut greenhouse gas emissions are considerably higher than that.”13

More ominously, Gernot Wagner and Martin Weitzman estimate that in the absence of significant policy change there is perhaps a one in ten chance that the globe’s average temperature will rise by 6°C or more, leading to “the end of the human adventure on this planet as we now know it.”14


One view, advocated most prominently by Bjorn Lomborg, holds that we should focus on other problems where the payoff might be greater. If we’re confident that the impact of climate change will not be catastrophic and/or irreversible, we should weigh costs versus benefits of various courses of action. Given that there are many other sources of death, bad quality of life, and unhappiness, perhaps we should direct our attention to those rather than climate change.15

Others, such as Martin Weitzman, conclude that, given the small but real possibility of catastrophic consequences from climate change, we should devote extraordinary effort to try to prevent it.16

Should we be willing to incur substantial cost in order to prevent an outcome that is very bad but highly uncertain? It may help to consider some other spheres where we have to make this kind of judgment. One is our criminal justice system, where we want to avoid wrongly convicting an innocent person. If a juror thinks there is reasonable doubt that the accused is guilty, she or he must vote “not guilty,” even though this means we will inevitably let some guilty people go free. Another is terrorist attacks and catastrophic disease outbreaks. Although the probability of these occurring is relatively low, we take extraordinary and costly measures in order to prevent them.

How much will it cost us to prevent significant climate change? An estimate by the Congressional Budget Office puts the cost of an aggressive carbon-reduction effort at 1-3% of GDP.17 Yet not acting might be just as costly. According to one respected estimate, by William Nordhaus, the loss from a rise in temperature of 2.5°C would be about 2% of global GDP.18

How can we do it? First, we can reduce greenhouse gas emissions. The United States accounts for about one-fifth of worldwide carbon emissions, so we could help a lot even if we act alone.19 This can be done via a carbon tax or a cap-and-trade program. Sweden and Norway have had a carbon tax since the early 1990s, and the Canadian province of British Columbia has had one since 2008. America used cap-and-trade programs to successfully deal with leaded gasoline in the 1980s and acid rain pollution in the 1990s. California began a cap-and-trade program to reduce greenhouse gas emissions in 2013, part of a broader effort to reduce emissions in the state to 40% below 1990 levels by 2030.20 Quebec and Ontario also have cap-and-trade programs for greenhouse gases. To this point, however, congressional Republicans have staunchly opposed federal government action in the form of either a carbon tax or a cap-and-trade program.21

Even better would be an international agreement that reduces emissions in other countries too. Efforts over the past several decades have been discouraging, though the 2015 Paris Climate Accord offers some grounds for optimism.22 An alternative to voluntary agreements, which are vulnerable to free riding, is the idea of a “climate club.” According to William Nordhaus, “Under the club rules, participating countries would undertake harmonized but costly emissions reductions…. Countries who are outside the club — and do not share in the burden of emissions reductions — are penalized…. Economic modeling indicates that the most promising penalty is uniform percentage targets on the imports of nonparticipants into the club region. A country considering whether to undertake costly abatement would have to weigh those costs against the potentially larger costs of reduced trade with countries in the club.”23

How much reduction in greenhouse gas emissions do we need? Many climate scientists believe we should aim for an increase in average temperature of no more than 2°C. Here is a statement by one knowledgable observer of how to achieve that: “Roughly speaking, to have a significant chance — greater than 50% — of keeping total warming below 2°C, we need to cut the emissions of carbon dioxide and other major greenhouse gas pollutants by more than 50% by mid-century, which in turn means that global greenhouse gas emissions must peak within a decade or so and start a rapid decline. That decline must continue through century’s end so that by 2100, the world’s total net emissions of greenhouse gases needs to be close to zero, and preferably below zero, especially if we delay serious action much longer. The goal is to stabilize atmospheric concentrations of carbon dioxide below 450 parts per million.”24

Second, we can promote clean energy alternatives, so that we’re not simply reducing use of dirty (climate-change-accelerating) energy but hastening its replacement. The chief recommendation here is to invest in research on clean energy.25

A third possibility is to try to alter the earth and/or its biosphere in ways that counteract the impact of climate change. Strategies include removing carbon from the air and injecting large quantities of aerosols into the stratosphere to deflect more sunlight away from the planet. In a pair of reports issued in 2015, the US National Academy of Sciences concluded that while such efforts are worth further exploration, “There is no substitute for dramatic reductions in the emissions of carbon dioxide and other greenhouse gases to mitigate the negative consequences of climate change.”26


As figures 9-12 show, around two-thirds of Americans believe most scientists agree global warming is occurring, believe climate change is due to human activities, worry a great deal or a fair amount about climate change, and feel there should be stricter laws and regulations to protect the environment. There is an enormous difference in these views between Americans who identify as Democrats and those who identify as Republicans.

Figure 9. Most scientists believe global warming is occurring
Estimated share of US adults. Question: “Which one of the following statements do you think is most accurate: most scientists believe that global warming is occurring, most scientists believe that global warming is not occurring, or most scientists are unsure about whether global warming is occurring or not?” “Democrat”: Democrat or lean Democrat. “Republican”: Republican or lean Republican. Data source: Gallup, “Gallup Poll Social Series: Environment,” 2016.

Figure 10. Climate change is due more to human activities than to natural changes
Estimated share of US adults. Question: “From what you have heard or read, do you believe increases in the Earth’s temperature over the last century are due more to the effects of pollution from human activities or to natural changes in the environment that are not due to human activities?” “Democrat”: Democrat or lean Democrat. “Republican”: Republican or lean Republican. Data source: Gallup, “Gallup Poll Social Series: Environment,” 2016.

Figure 11. I worry about climate change a great deal or a fair amount
Estimated share of US adults. Question: “How much do you personally worry about climate change/global warming?” Response options: a great deal, a fair amount, only a little, not at all. “Democrat”: Democrat or lean Democrat. “Republican”: Republican or lean Republican. Data source: Gallup, “Gallup Poll Social Series: Environment,” 2016.

Figure 12. There needs to be stricter laws and regulations to protect the environment
Estimated share of US adults. Question: “Do you agree or disagree: There needs to be stricter laws and regulations to protect the environment.” “Democrat”: Democrat or lean Democrat. “Republican”: Republican or lean Republican. Data source: Pew Research Center, Question Database, American Values Survey, q40a.

Figure 13 shows that only half of Americans believe rising temperatures are extremely or very dangerous. Although low, this share isn’t exceptional compared to other rich democratic nations.

Figure 13. A rise in the world’s temperature caused by climate change is extremely or very dangerous
Estimated share of adults. Question: “In general, do you think that a rise in the world’s temperature caused by climate change is extremely dangerous for the environment, very dangerous, somewhat dangerous, not very dangerous, or not dangerous at all?” Data source: International Social Survey Programme (ISSP), “Environment III,” 2010,, variable 43 q14e.


The hypothesis that greenhouse gas emissions are causing climate change is compelling. The evidence in support of the hypothesis is strong, and there is near-consensus among experts about this.

There is less agreement about how aggressive we should be in trying to slow the process, due in part to differing views about how we should weight the real but seemingly small possibility that the outcome will be catastrophic.

  1. Carbon Dioxide Information Analysis Center, “800,000-year Ice-Core Records of Atmospheric Carbon Dioxide”; Earth System Research Laboratory, National Oceanic and Atmospheric Administration, “Trends in Atmospheric Carbon Dioxide”; Intergovernmental Panel on Climate Change (IPCC), “Working Group 1 Summary for Policymakers,” Fifth Assessment Report, 2013, p. 7; Wikipedia, “Carbon Dioxide in Earth’s Atmosphere.”  
  2. P.J. Gerber et al, Tackling Climate Change Through Livestock: A Global Assessment of Emissions and Mitigation Opportunities, Food and Agriculture Organization of the United Nations, 2013. See also Natasha Gilbert, “One-Third of Our Greenhouse Gas Emissions Come from Agriculture,” Nature, 2012; Rob Bailey, Antony Froggatt and Laura Wellesley, “Livestock: Climate Change’s Forgotten Sector,” Research Paper, Chatham House, 2014; J. Poore and T. Nemecek, Reducing Food’s Environmental Impacts Through Producers and Consumers,” Science, 2018; Ecofys, “World Greenhouse Gas Emissions Flow Chart”. 
  3. Julia Moskin, Brad Plumer, Rebecca Lieberman, and Eden Weingart, “Your Questions About Food and Climate Change, Answered,” New York Times, 2019. 
  4. Wikipedia, “Atmospheric Methane.”  
  5. R. Rohde, R.A. Muller, R. Jacobsen, E. Muller, S. Perlmutter, et al, “A New Estimate of the Average Earth Surface Land Temperature Spanning 1753 to 2011,” Geoinformatics and Geostatistics: An Overview, 2013. 
  6. “The Science of Climate Change,” The Economist, 2010; C. Wickham, R. Rohde, R.A. Muller, J. Wurtele, J. Curry, et al., “Influence of Urban Heating on the Global Temperature Land Average using Rural Sites Identified from MODIS Classifications,” Geoinformatics and Geostatistics: An Overview, 2013. 
  7. Joseph Romm, Climate Change: What Everyone Needs to Know, Oxford University Press, 2016, p. 10. 
  8. IPCC, “Working Group 1 Summary for Policymakers,” Fifth Assessment Report, pp. 12; Justin Gillis, “Climate Panel Cites Near Certainty on Warming,” New York Times, August 20, 2013. William Nordhaus points out that “Scientists are increasingly confident that the basic results of climate modeling are accurate. Climate models calculate that past emissions have contributed to warming of almost one degree centigrade over the last century, with rapid continued warming projected over the present century and beyond. In its 2001 report, the Intergovernmental Panel on Climate Change reported that human activity was ‘likely’ to be the source of this warming. The IPCC upgraded this evaluation to ‘very likely’ in its 2007 report and to ‘extremely likely’ in its 2013 report.” Nordhaus, “A New Solution: The Climate Club,” New York Review of Books, 2015 
  9. William R.L. Anderegg, James W. Prall, Jacob Harold, and Stephen H. Schneider, “Expert Credibility in Climate Change,” Proceedings of the National Academy of Sciences, 2010. 
  10. John Cook et al, “Quantifying the Consensus on Anthropogenic Global Warming in the Scientific Literature,” Environmental Research Letters, 2013. 
  11. Climate Change Panel, American Association for the Advancement of Science, What We Know: The Reality, Risks, and Response to Climate Change, 2014, p. 2. 
  12. Romm, Climate Change. 
  13. IPCC, “Working Group 1 Summary for Policymakers,” Fifth Assessment Report, pp. 18, 21; Potsdam Institute for Climate Impact Research and Climate Analytics, “Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided,” The World Bank, 2012; Romm, Climate Change, p. 94. 
  14. Gernot Wagner and Martin L. Weitzman, Climate Shock: The Economic Consequences of a Hotter Planet, Princeton University Press, 2015, ch. 3. See also Steven C. Sherwood and Matthew Huber, “An Adaptability Limit to Global Warming Due to Heat Stress,” Proceedings of the National Academy of Sciences, 2010. 
  15. Bjorn Lomborg, Cool It, Random House, 2007; “The Climate May Be Heating Up Less in Response to Greenhouse-Gas Emissions Than Was Once Thought,” The Economist, 2013. 
  16. Martin L. Weitzman, “Fat-Tailed Uncertainty in the Economics of Catastrophic Climate Change,” Review of Environmental Economics and Policy, 2011; Gernot and Weitzman, Climate Shock. 
  17. Congressional Budget Office, “Effects of a Carbon Tax on the Economy and the Environment,” 2013. 
  18. Paul Krugman, “Building a Green Economy,” New York Times, 2010; William D. Nordhaus, “Why the Global Warming Skeptics Are Wrong,” New York Review of Books, 2012. 
  19. Environmental Protection Agency, “Global Greenhouse Gas Emissions Data.” 
  20. Center for Climate and Energy Solutions, “California Cap-and-Trace”; Jennifer Medina and Matt Richtel, “Carbon Goal in California Is ‘Milestone’ on Climate,” New York Times, 2016; Brad Plumer, “How California Plans to Go Far Beyond Any Other State on Climate,” New York Times, 2017. 
  21. “Harry Reid: Senate Will Abandon Cap-and-Trade Energy Reform,” Christian Science Monitor, 2010. 
  22. “Climate Talks End with Modest Deal on Emissions,” New York Times, 2010; “Nations Approve Landmark Climate Accord in Paris,” New York Times, 2015. 
  23. Nordhaus, “A New Solution: The Climate Club.” 
  24. Romm, Climate Change, p. 154. 
  25. Ted Norhaus and Michael Shellenberger, Break Through, Houghton Mifflin, 2007; Jim Manzi, “Keeping Our Cool: What to Do about Global Warming,” Cato Unbound, 2008; Romm, Climate Change. 
  26. National Research Council, Committee on Geoengineering Climate, “News Release,” 2015.