How much CO2 at 1.5°C and 2°C?

The Paris Agreement in 2015 was a step-change in international ambition to avoid the most dangerous impacts of climate change, setting targets to limit the rise in global temperatures to well below 2 °C above the pre-industrial era, and to strive for 1.5 °C. However, even these global warming levels will represent significant environmental changes, and will require some level of adaptation.

At 1.5 °C or 2 °C above pre-industrial temperatures, the concentration of carbon dioxide (CO₂) in the atmosphere will be higher than it is today, with impacts on land and marine ecosystems, and on food production. A new paper by Richard Betts and Doug McNeall, published in Nature Climate Change, quantifies the possible range of atmospheric CO₂ concentrations that would occur if global temperatures reach these global warming levels. The paper argues that risk assessment and adaptation planning for 1.5 and 2 °C warming may need to consider the biological impacts of a wider range of CO₂ concentrations than those provided by current climate model projections.

Environmental impacts of CO₂

Higher CO₂ promotes vegetation growth, but the extent of this fertilisation effect varies widely between species; can alter their ecological competitiveness; and can reduce the nutrient content of food crops. In the oceans, increased uptake of CO₂ causes ocean acidification with widespread impacts on marine ecosystems, particularly calcifying organisms such as corals. It is important to know the future concentration of CO₂ to determine the extent of these environmental impacts, and to fully understand the benefits of stringent mitigation action to limit the rise in global temperatures to 1.5 °C.

A range of possible CO₂ levels in a 1.5 °C or a 2 °C world

Betts and McNeall infer a range of possible CO₂ levels, related to a range of published climate sensitivity estimates. If global warming of 1.5 or 2 °C is realised at relatively low levels of CO₂ (i.e. high climate sensitivity), the direct impacts of CO₂ on plant photosynthesis and ocean acidification etc will be relatively small. In contrast, the direct effects of CO₂ could be far greater if climate sensitivity is low, as CO₂ will be higher for a given warming level.

One common index of climate sensitivity is the Transient Climate Response (TCR), which is the estimated level of global warming at the time of CO₂ doubling, following a rise of 1% per year. To quantify the uncertainties in CO₂-equivalent (CO₂e) concentrations (i.e. including all radiative forcing agents) that would occur if global warming crosses 1.5 °C and 2 °C, two different ranges of TCR are used. One of these is taken from the latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5), and the other from a more recent paper (Richardson et al., 2016; R16) that accounts for the discrepancy between models and observations.

Based on the TCR estimated from observational constraints in IPCC AR5, this paper estimates median CO₂ concentrations at 1.5 and 2 °C of 507 ppm and 618 ppm, respectively. The 5-95% confidence ranges are 425-785 ppm for 1.5 °C and 489-1106 ppm for 2 °C. Median estimates based on Richardson et al. (2016) imply a slightly higher CO₂e concentration at either temperature compared to AR5 (523 and 644 ppm for 1.5 and 2 °C, respectively), but the 5-95% ranges are lower (385-765ppm and 429-1069 ppm).

It is important to note that the range of CO₂-e concentrations presented in this paper are relevant for impacts studies at the time that global warming levels of 1.5 or 2 °C are first reached. They do not prescribe CO₂ levels compatible with keeping warming below the long-term targets of the Paris Agreement.

Why is this range important?

The CO₂-equivalent concentrations projected in the CMIP5 multi-model ensemble span much smaller ranges than those presented in this paper. In fact, for CO₂e at 2°C, the CMIP5 models cover just over half of the 5%-95% range estimated in this study, omitting the upper third. This indicates that the CMIP5 projections may not represent the possibility of substantially higher CO₂ concentrations. The range of relative strengths of direct CO₂ effects on photosynthesis, plant water use efficiency and ocean acidification are therefore not fully accounted for by these multi-model projections.

If the full set of effects of CO₂ on land and marine ecosystems and food production when passing 1.5 °C and 2 °C are to be assessed, a wider range of CO₂ concentrations for each level of global warming should be considered. This would provide a more complete picture of biological impacts when undertaking risk assessment and adaptation planning for scenarios that exceed 1.5 °C and 2 °C global warming. 

References

IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp. Huntington C, Cox P et al.

Richardson, M., Cowtan, K., Hawkins, E., Stolpe, M. B. (2016). Reconciled climate response estimates from climate models and the energy budget of Earth. Nat. Clim. Chang. 6, 931–935. doi:10.1038/nclimate3066