IN BRIEF: EFFECTS OF SHORT-LIVED (SOME HFCS) AND LONG-LIVED GASES

Long-lived greenhouse gases accumulate in the atmosphere, so continued emissions of these gases lead to continually increasing warming. Warming created by long-lived gases is not naturally reversible on the timescale of decades-to-centuries. Short-lived greenhouse gases affect the climate in qualitatively different ways to CO2, with constant rates of emission leading to an approximately constant level of raised global average temperature but not continually increasing warming.

Long-Lived Greenhouse Gases
CO2 is the main GHG emitted by humans:

• Once CO2 is emitted the land surface and the ocean take up some carbon out of the atmosphere, but a significant fraction remains for centuries to millennia.
• This creates warming that persists in the long-term. Each additional tonne of CO2 emitted adds more long-lasting CO2 to the atmosphere and creates more warming, meaning that global temperature increases in proportion to the cumulative total emissions of CO2.
• Non-CO2 GHGs with long lifetimes in the atmosphere affect the climate in similar ways to CO2. Nitrous oxide (N2O lifetime of ~120 years) and a number of F-gases (e.g. SF6 with a lifetime of 3,200 years) fall into this category.

Long-lived greenhouse gases accumulate in the atmosphere, so continued emissions of these gases lead to continually increasing warming. Warming created by long-lived gases is not naturally reversible on the timescale of decades-to-centuries. Therefore, reducing this warming requires the removal of long-lived gases from the atmosphere. CO2 and other long-lived gases can be aggregated as 'CO2 equivalent' whilst still relatively accurately capturing their effects on global temperature.

Short-Lived Greenhouse Gases
Greenhouse Gases (GHGs) that are much shorter-lived than CO2 such as methane - CH4 (lifetime ~12 years) and some HFCs such as HFC-32 (lifetime ~4.9 years) and HFC-134a (lifetime ~14 years) behave very differently:

• The effect of these short-lived GHGs on global average temperature is much more closely controlled by their emissions rate as opposed to the cumulative total of emissions over time.
• Their relatively short-lifetimes mean that for a constant rate of emission, atmospheric concentrations of a short-lived gas quickly increase to the point at which the amount of the gas decaying out of the atmosphere each year is equal to the amount being added through new emissions, keeping atmospheric concentrations constant. This only causes a slow increase in global temperature as the deep oceans warm-up on the timescale of several centuries. Maintaining constant emissions of a short-lived gas would therefore maintain the existing warming effect. This is unlike CO2, for which atmospheric concentrations and warming both continue to steadily increase with sustained emissions.
• Offsetting the slow additional increase in warming from short lived gases would only require emissions of the short-lived gas to fall by less than 1% per year. Reducing the rate of emissions of a short-lived gas faster than around 1% year would lead to lower atmospheric concentrations and a decrease in their effect on global warming.

Short-lived greenhouse gases affect the climate in qualitatively different ways to CO2, with constant rates of emission leading to an approximately constant level of raised global average temperature but not continually increasing warming. Aggregation as 'carbon dioxide equivalent' fails to capture this fundamental difference in how emissions of short-lived and long-lived GHGs affect global temperature. However international comparability supports the continued use of existing 'CO2 equivalence' metrics (the 100 year GWP).