How Do We Know that Humans Are the Major Cause of Global Warming?
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states: it is a greater than a 90 percent certainty that emissions of heat-trapping gases from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.” We all know that warming—and cooling—has happened in the past, and long before humans were around. Many factors (called “climate drivers”) can influence Earth’s climate—such as changes in the sun’s intensity and volcanic eruptions, as well as heat-trapping gases in the atmosphere.
So how do scientists know that today’s warming is primarily caused by humans putting too much carbon in the atmosphere when we burn coal, oil, and gas or cut down forests?
- There are human fingerprints on carbon overload. When humans burn coal, oil and gas (fossil fuels) to generate electricity or drive our cars, carbon dioxide is released into the atmosphere, where it traps heat. A carbon molecule that comes from fossil fuels and deforestation is “lighter” than the combined signal of those from other sources. As scientists measure the “weight” of carbon in the atmosphere over time they see a clear increase in the lighter molecules from fossil fuel and deforestation sources that correspond closely to the known trend in emissions.[2,3]
- Natural changes alone can’t explain the temperature changes we’ve seen. For a computer model to accurately project the future climate, scientists must first ensure that it accurately reproduces observed temperature changes. When the models include only recorded natural climate drivers—such as the sun’s intensity—the models cannot accurately reproduce the observed warming of the past half century. When human-induced climate drivers are also included in the models, then they accurately capture recent temperature increases in the atmosphere and in the oceans.[4,5,6] When all the natural and human-induced climate drivers are compared to one another, the dramatic accumulation of carbon from human sources is by far the largest climate change driver over the past half century.
- Lower-level atmosphere—which contains the carbon load—is expanding. The boundary between the lower atmosphere (troposphere) and the higher atmosphere (stratosphere) has shifted upward in recent decades. See the ozone FAQ for a figure illustrating the layers of the atmosphere.[6,7,8] This boundary has likely changed because heat-trapping gases accumulate in the lower atmosphere and that atmospheric layer expands as it heats up (much like warming the air in a balloon). And because less heat is escaping into the higher atmosphere, it is likely cooling. This differential would not occur if the sun was the sole climate driver, as solar changes would warm both atmospheric layers, and certainly would not have warmed one while cooling the other.
Direct evidence of human contribution to atmospheric CO2
Carbon dioxide (CO2) is the main heat-trapping gas largely responsible for most of the average warming over the past several decades. To compare how CO2 stacks up in influence to the many other important heat-trapping gases contributing to climate change see the CO2 FAQ. There is a way that scientists can tease apart the atmospheric concentration of CO2 to see how much of the CO2 is from natural sources and how much is from combusted fossil fuel sources.
The atmospheric concentration of CO2 has increased from a pre-industrial era (AD 1000 – 1750) concentration of approximately 280 parts per million (ppm) to around 383 ppm, as measured at Mauna Loa, Hawaii in 2007.[2,9] The carbon in the atmospheric CO2 contains information about its source, so that scientists can tell that fossil fuel emissions comprise the largest source of the increase since the pre-industrial era.
Here’s how scientists know. The same elements (i.e. same number of protons in the nucleus) with different mass numbers (arising from the different numbers of neutrons in the nucleus) are called isotopes. Each carbon molecule has six protons in the nucleus, but there are many different isotopes with varying numbers of neutrons in the nucleus. Carbon isotopes from different sources are “lighter” (high negative value) or heavier (lower negative value). For example, carbon from ocean is the standard with a value of “0” while carbon from fossil fuels ranges from -20 to -32. While atmospheric carbon has an average value of -5 to -9, it is becoming “lighter” over time as carbon from fossil fuels become more abundant in the atmosphere (Figure 1).[9,11,12]
Figure 1. Direct Evidence of Fossil Fuel Derived CO2 in the Atmosphere
The combination of natural drivers plus human drivers best match reality
(1) Natural and Human Factors that Influence the Climate (known as “climate drivers”)
Many natural and human factors influence climate. Natural factors include the energy from the sun; periodic volcanic eruptions of tiny particles, dust, and salt spray—all known as aerosols— many that can reflect sunlight; and natural carbon cycle processes such as termite mounds in Africa that emit methane or tiny organisms in the ocean surface that take up carbon dioxide. Human climate drivers include heat-trapping emissions from burning coal, gas and oil in power plants and cars; cutting down and burning forests; tiny pollution particles known as aerosols; black carbon pollution more commonly referred to as soot; and changes in land use that change how much the Earth’s surface reflects the sun’s energy back into space (referred to as albedo).
Some of these climate drivers result in net warming and others lead to cooling, but all are usually expressed as Radiative Forcing (RF) in units of watts per square meter. When all the natural and human-induced climate drivers are stacked up and compared to one another, the accumulation of human-released heat-trapping gases in the atmosphere is so large that it has very likely swamped other climate drivers over the past half century, leading to observed global warming (see Figure 2).[2,4,5]
Figure 2. Twentieth Century History of Climate Drivers
Heat-trapping emissions (greenhouse gases) far outweigh the effects of other drivers acting on Earth’s climate. Source: Hansen et al. 2005, Figure adapted by Union of Concerned Scientists 
(2) Cooling that partially offsets recent warming: “Global Dimming”
The figure above also depicts the sharp cooling influence a large volcanic eruption can have as it spews tiny particles high into the stratosphere (the layer of the atmosphere above the troposphere where weather typically occurs). The massive explosions from Krakatoa (Indonesia) in 1883 and Mount Pinatubo (Philippines) in 1991, for example, can be seen as the two largest downward spikes in the blue volcanic data depicted in the figure. These particles prevented the full energy of the sun from reaching the surface of Earth and thus created a cooling trend for several years.
Fossil fuel burning by humans also emits tiny particles. Some particles reflect sunlight back to space (aerosols), similar to the volcanic particles; other pollution particles such as soot (black carbon) absorb the sunlight, which leads to local warming of the atmosphere level where the soot particles circulate (see Aerosols FAQ). Both types of human-created particles lead to a decrease in the amount of sun’s energy reaching the surface of Earth. The term “global dimming” has been used to describe this phenomenon. There very likely would have been even more warming in the past 60 years if it were not for these human-made and natural tiny particles.
(3) Observed temperature changes on land and ocean compared with natural and human climate drivers
The IPCC has carefully documented observed changes in air temperature, ocean temperature, ice retreat, and sea level rise over the past century. These observed changes are then compared in climate computer models with natural climate drivers and human climate drivers. The IPCC concluded that the observed changes are unlikely to be the result solely of natural processes. [4,6] Figures 3 and 4 illustrate several examples from climate models that show that the combination of natural and human climate drivers closely match observed historical trends. Conversely, the corresponding figures from climate models using only natural climate drivers do not recreate the observed trends very well.
Figure 3. Combined Natural Plus Human Drivers Best Match Observed Atmospheric Temperature
Figure 4. Combined Natural Plus Human Drivers Best Match Observed Ocean Temperature
The boundary between the lower atmosphere (troposphere) and the upper atmosphere (stratosphere) has shifted upward in recent decades. This boundary is called the tropopause. Similar to the cases outlined above, models that have both natural and human climate drivers match the observed change in tropopause height better than models with only natural climate drivers.
The likely cause of the rise in tropopause height is from heat-trapping gases accumulating in and heating up the troposphere and conversely blocking heat from getting into the stratosphere, thus causing cooling there.[6,7,8] Decreased ozone in the stratosphere also adds to this cooling stratosphere trend (see ozone FAQ.) This would not occur if the sun was the sole climate driver, as solar changes would have warmed both the stratosphere and the troposphere.
One way to think about this is how when air of a hot air balloon heats up it expands and the top of the balloon rises. The same general idea can be applied here—the troposphere volume expands and rises as it warms, since the boundary between the troposphere and the stratosphere is in part defined by a change in temperature.
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Summary prepared by B. Ekwurzel and reviewed by N. Cole, P. Frumhoff, and S. Shaw (UCS).