Where are we now? Where are we going? How will we know if we get there?
A carbon budget implies that there is a limit on the total amount of global CO2 emissions we can produce in order to halt warming at a certain level, such as the 1.5 or 2.0 °C set out in the Paris Agreement.
The Paris Agreement is usually interpreted in terms of human-induced warming, due to anthropogenic emissions of CO2 and other greenhouse gases.
However, the warming record is subject not only to anthropogenic influences, but also natural fluctuations in the climate system (such as changing weather patterns or El Niño oscillations that act on multi-decadal time scales), which are referred to as internal climate variability.
So how will we know if we exceed the Paris Agreement warming targets?
One way of estimating the anthropogenic-only (noise-free) component of warming is by the Anthropogenic Warming Index (AWI). However, the AWI is also subject to assumptions about uncertainty due to internal variability and how it is removed.
Alternatively, in the climate-model world, we can run a climate model multiple times with small differences in initial conditions, producing a large ensemble of results. By taking a mean of all those possible climates with different initial conditions, we can estimate the externally-forced response (natural variability averages out among the different ensemble members of the same climate model).
A recent study led by CONSTRAIN researchers, published in Environmental Research Letters, discusses the uncertainty in carbon budget estimates due to internal climate variability and compares different approaches for estimating the remaining carbon budgets for 1.5 or 2.0 °C of human-induced warming.
What are the key findings?
Due to the intrinsic uncertainty arising from internal variability in the climate system, it may only be possible to determine the exact year when a carbon budget is exceeded with hindsight. This highlights the importance of a precautionary approach.
Why is this important?
These results do not question the validity of using a carbon budget approach to determine the level of mitigation needed to stay within a global temperature limit.
However, it does mean that caution is needed when interpreting small remaining carbon budgets, since internal climate variability presents yet another source of uncertainty.
The full paper is: Uncertainty in carbon budget estimates due to internal climate variability by Katarzyna Tokarska et al., Environmental Research Letters (2020).