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. 


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