The Closing Door of Climate Targets

Robust evidence from a range of climate–carbon cycle models shows that the maximum warming relative to pre-industrial times caused by the emissions of carbon dioxide is nearly proportional to the total amount of emitted anthropogenic carbon. This proportionality is a reasonable approximation for simulations covering many emissions scenarios for the time frame 1750 to 2500. This linear relationship is remarkable given the different complexities of the models and the wide range of emissions scenarios considered. It has direct implications for the possibility of achieving internationally agreed climate targets.

The considerations presented here are based on the assumption of a generic set of carbon dioxide emissions scenarios that reasonably approximate what is presently observed and what needs to be done to limit warming below a specific global mean temperature increase. In these idealized and illustrative emissions scenarios, emissions follow an exponential increase with a constant rate until a given year, after which the emissions decrease exponentially at a constant rate. The scenarios delineate the boundaries for any discussion and decision process for global measures limiting anthropogenic climate change.

For example, under these assumptions, keeping CO₂-induced global warming below 2°C would require emissions reductions of almost 3.2% per year from 2020 onward; this is more than doubled if Global mitigation doesn’t begin until 2032. So every year counts; the longer the delay, the more reductions are required later.

Without mitigation of carbon dioxide, it becomes impossible to reach lower temperatures, the climate target closes irreversibly.

Under the present illustrative assumptions, the 1.5°C target expires after 2028, and the 2°C target vanishes after 2044. These times would be later if a period of stabilized emissions preceded the Global Mitigation Strategy (GMS). The more likely situation, however, is that a specific climate target becomes unreachable much earlier, because there are upper limits on sustained emissions reduction rates imposed by what the countries’ economies can realize collectively given the present state of technology and infrastructure.

Economic models estimate that feasible maximum rates of emissions reduction may not exceed about 5% per year (5). Under this assumption, the 1.5°C target has become unachievable before 2012, the 2°C target will become unachievable after 2027, and the 2.5°C target will become unreachable after 2040.

These years are only illustrative of the finite time that climate targets remain available options in the presence of continued greenhouse gas emissions.

As the emissions scenarios considered here illustrate, even well-intentioned and effective international efforts to limit climate change must face the hard physical reality of certain temperature targets that can no longer be achieved if too much carbon has already been emitted to the atmosphere. Both delay and insufficient mitigation efforts close the door on limiting global mean warming permanently. This constitutes more than a climate change commitment: It is the fast and irreversible shrinking, and eventual disappearance, of the mitigation options with every year of increasing greenhouse gas emissions.