Two or 12 degrees warmer? Greenhouse gas emission scenarios that drive future climate outlooks

It’s 2100. Thousands of power plants incinerate coal to help feed a bustling world economy. People from Bangladesh to the United States generally are more affluent than they are today, and many of the planet’s 8.7 billion inhabitants can afford cars, air conditioning, and other comforts that make life easier but more resource intensive.

This combination of population growth and a burgeoning energy appetite come at a climate cost. Carbon dioxide (CO₂) spews from tailpipes and smokestacks at nearly four times the present rate, congesting the atmosphere with CO₂ concentrations that approach 1,000 parts per million (ppm)—nearly 350 percent more than the amount before the Industrial Revolution began in earnest in the late 1800s. The build-up of greenhouse gases has sent the global average temperature soaring up to 12 degrees Fahrenheit warmer than it is now.

Scientists have called the 1,000 ppm point and its impact on temperature the worstcase scenario, citing increased chances that about half of the known endangered species will go extinct, most regions of the globe will feel economic shocks, ocean acidification will eat away at coral reefs, and millions of people will be displaced as sea levels rise from melting polar and mountain ice. Many scientists have stated CO₂ should not exceed 450 ppm for a long period, if at all, to avoid dangerous warming of the planet.

The good news is that this future world picture represents only one of 40 scenarios that each project different climate futures; some include increases in temperature as low as 2 degrees F. The bad news is that between 2000 and 2008, global greenhouse gas emissions outpaced scientists’ worst-case scenario. So, what is the story with these scenarios, and how do scientists arrive at them?

Emission scenarios and temperatures projections

Future outlooks uniformly predict warmer temperatures but differ by up to 10 degrees. Whether temperatures in 2100 will be closer to 2 or 12 degrees hotter than present depends on two factors: realistically describing human actions that influence emissions of greenhouse gases, including economic development and population growth, and accurately modeling physical processes, such as air and water movement. While emission scenarios describe the atmospheric fallout of human actions, climate models use that information to simulate the response of physical processes that alter temperature and other climate variables.

Ultimately, greenhouse gases like CO₂ cause changes in temperature. However, scientists cannot estimate the extent of change without knowing the amount of gases injected into the atmosphere. To do this, the Intergovernmental Panel on Climate Change (IPCC) created six emission narratives to explore the evolution of and relationship among economic, social, and environmental factors that influence greenhouse gas emissions, and hence climate.

These story lines, published in 2000 in the Special Report on Emissions Scenarios (SRES), are conceptual outlines that guide quantitative assessments of the key factors that influence greenhouse gas emissions, including gross domestic product, income disparity, energy intensity, fossil fuel and clean energy use, and CO₂ consumption by the land. Six modeling groups used these guidelines to create different evolutions for the key indictors and generated 40 unique greenhouse gas scenarios, about seven for each narrative.

The groups did not cobble together haphazard assessments of these indicators. They were informed by more than 800 emission scenarios published in academic journals as well as sophisticated studies from institutions like the International Institute for Applied Systems Analysis, which projects population trends and highlights the strains that a more crowded planet place on resources and society.

The narratives, described in more detail in the “IPCC Emission Scenarios” textbox, unfold something like this: The A1T and B1 scenarios dump the least amount of greenhouse gases into the atmosphere. A1B and B2 have moderate emissions, and A1F1 and A2 spew the highest amount of gases. An outline of B1, for example, would describe a world in which solar and wind farms dot the landscape in many countries, population growth is low, and CO₂ emissions are slightly less than they were in 1990. On the other hand, in an A1FI world—“FI” stands for Fossil Fuel Intensive—thousands of coal-fired power plants would belch CO₂ and other greenhouse gases into the atmosphere at four times the present rate to fuel the economy.

Emission scenarios by themselves say little more about future climate than higher concentrations of greenhouse gases will elevate temperatures. To quantify gas concentrations in ppm, which allows for more precise estimates of temperature changes,scenarios are fed into state-of-the-art climate models. When the 40 emission scenarios are each filtered through climate models—which can capture thousands of dynamic natural processes that influence temperature, among other climate variables—the highest and lowest temperatures create bookends that provide a range in which global average temperature likely will fall.

The recently published Global Climate Change Impacts in the United States report conducted by the U.S. Global Change Research Program (GCRP) presents results from state-of-the-art climate models—the same ones used in the latest IPCC assessment report—driven by low (B1), high (A2), and higher (A1FI) emissions scenarios (Figure 1a).

For the B1 scenario, concentrations of CO₂ hover around 550 ppm by 2100, causing temperature increases of about 4 degrees F, according to the report. Concentrations of CO₂ and temperature in the A2 scenario climb even higher, reaching 850 ppm and about 7 degrees F. The A1FI scenario suggests CO₂ will approach 1,000 ppm, driving temperatures upwards of 8 degrees F (Figure 1b).

These results collectively suggest that if emissions are within the ranges specified by the narratives, a 66–90 percent chance exists that temperatures will be 3 to 8.5 degree F warmer than the 1960–1979 average. Lower probabilities, however, indicate a larger temperature range, between 2 and 12 degrees F. The results also show that temperatures for all of the story lines are similar until 2030, regardless of how society evolves.

Future Policy

The current scenarios should be viewed cautiously. They are not predictions, and the IPCC does not assign probabilities to their occurrences due to uncertainty. The genius of the scenarios is that they span a wide range of greenhouse gas emissions. With the help of climate models, this enables the exploration of the climate implications of each scenario for society and the planet.

The IPCC emissions scenarios also do not explicitly evaluate the effect of policy changes on emissions. For example, no scenario evaluates the impact of a worldwide adoption of the Kyoto Protocol, which puts enforceable limits on many greenhouse gases but was not ratified by some countries, including the United States. Also, the scenarios do not encompass the full range of possible emissions. The IPCC avoided “disaster” or “surprise” scenarios that describe economic collapse or crises that hurl society back to primitive times, or futures where emissions outpace the A1FI scenario.

New emission scenarios are in the works. They will include up-to-date data and a wider range of emissions. They also will likely include scenarios that analyze the impact of global climate treaties on greenhouse gas concentrations. Some will seek answers to the emission reductions needed to stabilize CO₂ concentrations at 450 ppm, 550 ppm, and other levels. The results will help test and devise new policy actions.

Many nations are debating legislation that curbs greenhouse gas emissions, and a world treaty which will update the Kyoto Protocol is set to be negotiated in December in Denmark. An important question is, to what degree will these new laws minimize temperature change? The U.S., for example, proposes reducing greenhouse gas emissions 17 percent below 2005 levels by 2020 and 83 percent by 2050. The European Union, which has the most aggressive targets, proposes a 30 percent cut below 1990 levels by 2020 and up to 80 percent by 2050.