Metrics Matter
The Earth’s energy balance – the balance between the inbound energy from the sun and outbound energy from the earth – is measured in units of watts per square meter (W/m2), called “radiative forcing” (RF). Rising temperatures, deepening droughts, intensifying storms, and other climate impacts are the result of an unprecedented human-caused disruption of this balance, leading to excessive heat, which is likewise measured in RF.
RF is measured at the top of the atmosphere, averaged over the 510 trillion square meters of the earth’s surface. As of 2019, the IPCC reported that RF had reached 2.72 W/m2, a level which, if sustained, will inevitably push global mean temperature above 2.0°C.
The degree to which each climate pollutant and other factors influences the Earth’s energy balance can be described in terms of positive or negative radiative forcing (RF). RF is also the basic metric used in IPCC modelling scenarios – “representative concentration pathway” (RCPs) and “shared socio-economic pathways” (SSPs).
By lowering radiative forcing (RF) sufficiently, we can turn the corner on climate change. This is like turning off the gas flame under a pot of boiling water. If we turn down the source of the heat, the temperature of the water will eventually drop back down. Thus, RF is a leading indicator of climate change, while temperature change is a lagging indicator. In short, turning around RF is the necessary path to stabilize our climate.
This is vital. By training our sights on the reduction of RF, we can begin to get ahead of changes in temperature and other impacts, instead of playing catch up. Our job now is to incentivize projects of sufficient scale to reduce and eventually draw down RF, and to do so with our eyes open to avoid causing unintended harm to people or the planet.
Key Climate Forcers (Excerpt from Table 2, Radiative Forcing Protocol)
| Climate Forcers Contributing to Positive RF | Climate Forcers Contributing to Negative RF |
|---|---|
| Well-mixed greenhouse gases | Well-mixed greenhouse gases |
| Carbon dioxide (CO2) | None |
| Nitrous oxide (N2O) | |
| Hydrofluorocarbons (HFCs) 1) | |
| Nitrous oxide (N2O) | |
| Chlorofluorocarbons (CFCs) | |
| Hydrofluorocarbons (HFCs) 1) | |
| Hydrochlorofluorocarbons (HCFCs) 1) | |
| Methane | |
| Non-well-mixed climate forcers | Non-well-mixed climate forcers |
| Tropospheric ozone (from non-methane precursors, including NOx, CO, and VOCs) | Mineral dust aerosols 2) |
| Black carbon | Nitrate aerosols |
| Brown carbon | Organic carbon |
| Mineral dust aerosols 2) | Sulfate aerosols |
| Sea salt aerosols | |
| Non-emission climate forcer | Non-emission climate forcer |
| Decrease in Albedo | Increase in Albedo |
| Waste heat | |
| 1) A few HFCs and HCFCs are non-well-mixed. 2) While mostly a negative climate forcer, mineral dust can also cause warming, depending on the iron and aluminum content and the particle size. | |