6 Why We Know It’s Fossil Fuels

We can combine radiative forcing from all the different factors: industrial-caused and natural, to create an estimate for radiative forcing as a function of time.

The global temperature since 1880 is plotted along with the best scientific estimate of the total radiative forcing from industrial pollution and natural factors (volcanoes, which make the air hazier after large eruptions, and solar variability). The industrial-caused radiative forcing has increased sharply in recent decades. With the exception of a few large volcanic events, El Chichón in 1982 and Pinatubo in 1991, the radiative forcing has been positive since the late 1970s. A positive radiative forcing results in the Earth heating up until the planet gets back into energy balance.

Global temperature anomaly in units of K, and radiative forcing in units of PW = 1000 TW (NASA GISS, NOAA, Inputs4MIPs, IPCC).

The radiative forcing is punctuated by large negative excursions due to volcanoes, and temperature tends to drop a little bit after these as well. Since the radiative forcing recovers quickly after a volcanic eruption, within about 2-3 years, so does the temperature.

Comparing the long-term trends in radiative forcing shows that the natural factors are tiny as compared with recent industrial-caused heat trapping. This is the most basic proof of how we know that global heating is caused by pollution. If you also consider the uncertainties in each forcing component (like air pollution aerosols in the previous section) and the potential for natural ups and downs of global temperature, we find that the industrial pollution explains somewhere between 70% and 130% of the warming. That is, natural factors are about equally likely to have caused cooling as they have warming. And the best estimate for the pollution-caused amount is 100%.

Have you ever heard the argument, “the climate is always changing, how do we know it’s fossil fuels now?” Let’s be clear about this: pollution is the cause. There’s no scientific evidence for any other cause of global heating except pollution.

CO₂-equivalent emissions

One way that is often used to put different pollutants on the same quantitative ground is carbon dioxide-equivalent emissions. Although there are drawbacks to this method, it is important to understand the details of the calculation.

First, one must decide on a timescale over which equivalence is to be measured. The standard for this is 100 years, but sometimes 20 or 500 years are chosen as alternatives.

Second, the global warming potential is calculated for the gas given the choice of timescale. Global warming potential compares the radiative forcing from the gas with that of carbon dioxide over the timescale. For example, over 100 years, methane traps 32 times as much heat as carbon dioxide. This means methane’s global warming potential is 32.

Since it’s defined as a ratio with carbon dioxide, the global warming potential of carbon dioxide is always 1. The table below has global warming potential values for some of the main heat-trapping gases using the 100 year timescale.

Source: IPCC.

To calculate CO₂-equivalent emissions, one multiplies the emissions of the gas by the global warming potential. CO₂-equivalent is still used as one method for reporting emissions on the international scale.

There are drawbacks to CO₂-equivalent however. Consider the temperature change from a pulse of methane and a pulse of CO₂. In order to do this calculation one has to make more assumptions: how quickly the ocean warms up, the climate sensitivity, and parameters for exchange between the surface and deep ocean. The parameter uncertainty is a reason why metrics like this aren’t considered as much as CO₂-equivalent. The temperature change from a 15 year pulse of CO₂ is compared with that of a 15 year pulse of an equivalent amount of methane is plotted below.

Temperature change from a 15 year pulse of methane compared with a pulse of 32 times as much carbon dioxide. These numbers were chosen so the CO₂-equivalent emissions (100 year timescale) would be the same. The time of zero years is when the pulse is turned off. 

The temperature change from methane is concentrated within the first few years and decreases substantially within a few decades. The temperature increase from carbon dioxide, though, lasts quite a long time, exceeding the methane value after 30 years and soon after becoming twice as large.

A 20 year timescale for global warming potential is often used to emphasize the importance of methane emissions even more. The results of the same experiment as above but using the same CO₂-equivalent over 20 years is below.

Temperature change from a 15 year pulse of methane compared with a pulse of 84 times as much carbon dioxide. These numbers were chosen so the CO₂-equivalent emissions (20 year timescale) would be the same. The time of zero years is when the pulse is turned off. 

This set-up shows an even more rapid overtaking of the CO₂-induced temperature change, after a decade. When considering the future warming effect, 20 year global warming potential is not a particularly good measure.

For a cessation of the long term temperature effect, we should hurry especially to reduce CO₂ emissions and other long-lived heat-trapping gases, even more than the CO₂-equivalent emissions suggest.