Why are scientists so confident that a business as usual future based on fossil fuels will lead to major changes in the Earth’s climate? Because we seek to understand climate in terms of cause and effect.
A very common misconception about climate change is that projections of future warming are based on extrapolation of recent warming trends. This misconception is fed by media reporting: both “fourth warmest January on record” and “global warming pause” narratives suggest that we’re waiting with bated breath to see what the climate will do, and whether emerging trends can be understood. Even well-intentioned science outreach often starts off with a graph showing rising temperatures as if this is the basis for our understanding and prediction.
But our expectations of future warming are not based on extrapolation of recent trends. Rather, we expect climate to be warmer in the future than in the past because we know that greenhouse gases absorb and then re-emit thermal radiation. As people around the world burn more and more fossil fuels, concentrations of greenhouse gases increase, so that solar energy accumulates under the extra absorbing gas. Scientists expect accumulating heat to cause warming temperatures because we know that when we add heat to things, they change their temperatures.
Earth’s climate results from a balance of energy flows into the planet (from the Sun) and out of the planet (by thermal infrared radiation) back to space. If more energy flows in than flows out, the Earth warms up. If more flows out than flows in, it cools off. This “cause and effect” framework is completely consistent with our everyday experience. More locally, it’s the reason day is warmer than night and summer is warmer than winter.
Instead of reacting to year-to-year changes in temperature as a measure of confidence in future climate change, a better way to understand global warming is by using the concept of climate sensitivity. In the “cause and effect” framework, changes in radiation are the cause (also called “forcing”) and changes in temperature or other variables are the effect (also called the “response”). Scientists measure climate forcing in Watts per square meter (W m-2) and of course changes in global mean temperature can be measured in degrees Celsius. The ratio of response to forcing is called the climate sensitivity. It’s convenient to express climate sensitivity in °C per W m-2, or more informally as “degrees per Watt.”
In my experience, expressing climate sensitivity in degrees per Watt is more helpful than the common usage of “degrees per doubling of CO2” because it emphasizes the causal relationship between heat and temperature. Even better, we can use this framing to understand climate changes in the past.
If the Earth was a simple ball of rock in space like the Moon, the climate sensitivity would be around 0.27 °C per W m-2, which is the amount of temperature difference required for rocks to change the rate they radiate heat by 1 W m-2. This number can be derived from the Stefan-Boltzmann Law but is also easy to measure in the laboratory, using rocks. But the real world is more complicated than a rock. It has clouds that reflect sunshine and trap nighttime cold, it has oceans and air that store and transport heat, and it has biology and chemistry that interact with the physical climate in fantastic ways. So to understand climate sensitivity of the real Earth, we examine how much warming and cooling have happened in the past and compare those responses to the forcing.
In 1991, when I was in graduate school, the largest volcanic eruption in recent memory occurred in The Philippines when Mount Pinatubo released enormous amounts of dust, ash, and sulfur dioxide. Besides producing lovely sunsets, the stratospheric particles from the Mount Pinatubo eruption measurably changed the amount of solar radiation reaching the Earth’s surface for a couple of years. Sunlight scattering off those stratospheric particles briefly produced a climate forcing of about 2.5 W m-2, with the effect decreasing to zero over about four years [Schmidt et al, 2012] as the volcanic particles settled out of the air. The loss of that little bit of solar heating caused the Earth’s climate to cool just a little bit in response. Global surface temperatures dropped by about 0.5 °C. Dividing the response of 0.5 °C by the forcing of 2.5 W m-2, we estimate the Earth’s climate sensitivity is about 0.20 degrees per Watt. Wigley et al (2005) estimate that this brief episode only allowed the climate to adjust about 30% of the way to its equilibrium response to the forcing. If the forcing lasts longer, the climate has longer to respond so sensitivity would then be around 0.2 / 0.3 = 0.7 degrees per Watt.
From about 900 to 1200 AD western Europe experienced relatively warm temperatures that allowed a flourishing of Medieval agriculture and economies. Norse expansion across the North Atlantic was at its height, with colonies established in Greenland and even the New World. This “Medieval Warm Period” was followed by gradual cooling over the next centuries, culminating in a “Little Ice Age” from about the 16th to the 19th Century. The Greenland colonies collapsed, European crop failures caused widespread famine, and glaciers advanced into Alpine villages. There’s some dispute about whether this was a truly global climate change because of course we don’t have reliable historical records for much of the world 1000 years ago, but proxy records suggest that there was at least some global cooling during this period.
What caused the climate to cool from the Medieval Warm Period to the Little Ice Age? Two important possibilities are an increase in the frequency of major volcanic eruptions during the 12th and 13th Centuries and a tendency toward less output by the Sun over this period. Volcanic eruptions are preserved as deposits of ash in cores recovered from the Greenland Ice Sheet. Solar activity has been reconstructed using sunspot counts and changes in isotopes formed by cosmic rays in the atmosphere. Taken together, these data suggest a total climate forcing of no more than 1 W m-2 (maybe less) [Schmidt et al, 2012]. Combining historical records, tree rings, lake sediments, and other proxies, the cooling over this period is estimated to be perhaps 0.8 °C [Mann et al, 2009]. Dividing climate response by climate forcing, we obtain a sensitivity of around 0.8 degrees per Watt, a little greater than the Pinatubo case.
The great granddaddy of all climate changes for which we have really good data is the dramatic warming that began about 18,000 years ago, at the end of the Last Ice Age. Over about the next 7000 years, the major ice sheets over North America and northern Europe collapsed, temperatures rose throughout the world, vegetation zones shifted poleward, and eventually agriculture emerged in the ancient world.
As the ice sheets melted, the albedo (brightness) of the land surface decreased, allowing more solar radiation to be absorbed. At the same time, atmospheric CO2 increased by about 55%, from 180 parts per million to about 280 parts per million. Using pollen records, algae deposited on the sea floor, and other proxy data, scientists have painstakingly reconstructed temperatures during this period. They find that the total climate forcing was about 6.5 W m-2, and the response was about 5 °C of warming. [Rohling et al, 2012]. Dividing the response by the forcing, we obtain an estimate of a little less than 0.8 degrees per Watt for climate sensitivity.
In each of the three cases above, we find that when there’s an imbalance between “heat in” and “heat out” there’s a corresponding change in Earth’s temperature. Furthermore, in all three cases, the response was about 0.7 to 0.8 degrees of warming or cooling per W m-2 of forcing. This is more than twice as strong as the “bare rock” response we’d get on the Moon, because the Earth apparently “amplifies” the initial forcing. A little cooling makes the air drier, for example, which reduces the greenhouse effect because water vapor is a powerful greenhouse gas. Similarly, a little warming melts some snow and ice so the Earth can absorb a little more sunshine. There are other past climate changes that can be used to estimate sensitivity as well, and they all broadly agree that the sensitivity of Earth’s climate is greater than would be the case for a bare rock. Knutti and Hegerl (2008) reviewed dozens of studies of climate sensitivity from both paleoclimate records and from physical principles, and find that the most likely value is about 0.8 °C per W m-2.
The reason scientists are so confident that Earth’s climate will warm in the 21st Century is that we know heat warms things up. Doubling CO2 concentration without warming the climate would reduce outgoing thermal radiation by 3.7 W m-2. If we have learned anything from Pinatubo, from the Last Millennium, or from the Ice Ages, it’s that when the Earth’s energy budget is out of balance, the climate changes. If climate warms by 0.8 °C per W m-2, then doubling CO2 should eventually warm the climate by (3.7 W m-2) x (0.8 °C per W m-2) = 3°C. This is about the same amount of warming predicted by climate models [Stocker et al, 2013], but you don’t need a climate model to get this number.
Unfortunately, if the world proceeds with economic development based on fossil fuels the CO2 will double not once but twice from preindustrial levels. That’s because out of 7 billion souls on this planet, only about 1 billion of us use a lot of energy. By the end of this Century another 3 billion people are projected to join our modern economy. Population is only expected to grow 30% by 2100, but energy use is expected to grow 300% [United Nations, 2004].
If all that energy is generated by burning stuff made of carbon, CO2 will quadruple from preindustrial values, adding not 3.7 Watts to each square meter but rather 7.4 W m-2. That’s ten times the difference form the Medieval Warm Period to the Little Ice Age. In fact, a business as usual future will change the Earth’s radiation balance by more than the change at the end of the Last Ice Age (7.4 vs. 6.5 W m-2). But while deglaciation took place gradually over 7000 years, we’re proposing to make that big a change 100 times as fast.
We expect that doubling CO2 and doubling it again will cause dramatic climate change for the same reason we expect day to be warmer than night, summer warmer than winter, and Miami warmer than Minneapolis. It’s not because of computer models, and it’s not because this January was the fourth warmest on record. It’s because when you add heat to things, they change their temperature.
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