EDITOR'S NOTE 11/18/13: After this blog post was published, the IPCC updated its Summary for Policymakers. The figures in this blog post have been updated to reflect new information.

The Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report (AR5) has delivered an overwhelming consensus that climate change impacts are accelerating, fueled by human-caused emissions. We may have just about 30 years left until the world’s carbon budget is spent if we want a likely chance of limiting warming to 2 degrees C. Breaching this limit would put the world at increased risk of forest fires, coral bleaching, higher sea level rise, and other dangerous impacts.

When Will Our Carbon Budget Run Out?

The international community has adopted a goal for global warming not to rise above 2°C compared to pre-industrial temperatures. Scientists have devoted considerable effort to understanding what magnitude of emissions reductions are necessary to limit warming to this level, as the world faces increasingly dangerous climate change impacts with every degree of warming (see Box 1).

IPCC AR5 summarizes the scientific literature and estimates that cumulative carbon dioxide emissions related to human activities need to be limited to 1 trillion tonnes C (1000 PgC) since the beginning of the industrial revolution if we are to have a likely chance of limiting warming to 2°C. This is “our carbon budget” – the same concept as a checking account. When we’ve spent it all, there’s no more money (and the planet’s overdraft fees will be much more significant than a bank’s small charges for bounced checks).1

The report also states that as of 2011, we have emitted roughly 515 PgC since the industrial revolution, meaning we have already burned through about 52 percent of that carbon budget.

Do the math, and the world only has 485 PgC left in the budget. This balance puts us on track to exhaust our remaining carbon budget before the end of 2045 under a carbon intensive trajectory.2

For context, consider Earth’s increasing pace of emissions: While the first half of the entire global carbon budget was used up over 250 years, the second half of the budget would be used up in only about three decades if emissions continue unabated.

This new finding has significant implications for how many more years we can continue to emit at a carbon-intensive rate. In this post, I’ll explain why the 1 trillion tonne figure and the three decade estimate matter.

Why Is the IPCC Talking About Cumulative Emissions?

To understand emissions reductions necessary to have a good chance of limiting warming to 2°C, the climate community has focused largely on emissions pathways – that is, when greenhouse gas emissions peak and the rate at which they must decline (e.g. peak sooner and then reduce less steeply versus peak later and then reduce more steeply).

But given advances in coupled climate-carbon models, a growing number of scientific papers3 have shown temperature is closely related to the total amount of CO2 emissions released over a time period – cumulative emissions – rather than the timing of those emissions. Taken another way, from a climate perspective, the total area under the emissions trajectory matters more for peak warming than the shape of that curve.

As emissions increase, a number of intervening processes occur which essentially cancel each other out, leading to the approximately linear relationship between warming and cumulative emissions. For example, as atmospheric concentrations of CO2 increase, every tonne of carbon dioxide has less of an effect on warming (the strongest absorption bands are already saturated). However, many carbon sinks (e.g. the ocean) become less effective at absorbing carbon dioxide at higher concentrations of dissolved CO2. These effects roughly balance each other out, leading to a direct relationship between the total amount of CO2 emitted over a given time period and warming.

Should the Next Set of Commitments Under the UN Framework Convention on Climate Change (UNFCCC) Be Based on Cumulative Emissions?

It is critically important for the international community to keep in mind the total carbon budget when designing the next set of emissions-reduction commitments. Otherwise, targets can be made for certain points in time (e.g. 2030) without planning to limit the overall amount of emissions that build up in the years in between.

Box 1: The Impacts of a 2°C World

  • Roughly 2.6 feet of sea level rise above 1980-99 by the end of the century

  • Average annual runoff decreasing 20-40 percent in the Danube, Mississippi, Amazon and Murray Darling river basins

  • Average annual runoff increasing about 20 percent in the Nile and Ganges basins

  • Forest fires almost doubling by 2050 in Amazonia with 1.5°C to 2°C above pre-industrial levels

  • The risk of crossing thresholds in tipping points in the Earth system (e.g. West Antarctic ice sheet disintegration and Amazon dieback) increase

  • The frequency of bleaching events exceeds ability of coral reefs to recover

For example, many countries have already established 2020 emissions reduction goals without accounting for the overall carbon budget. This means emissions could rise until shortly before 2020 and then drop quickly. While the target would still be met, larger cumulative emissions would result than if emissions had steadily declined over the same period.

Yet the new research does not mean decision-makers should give up on timetable-based emissions-reduction targets. For example, Meinshausen et al. showed the higher emissions are in 2020, the more challenging it will be to limit warming to 2°C. If emissions peak later, the rates of decline afterwards become quite steep.

Hedging on Rapid Decarbonization Is No Safe Bet

Betting our carbon budget on a fast decline in emissions later will not only be costly, but may be technologically unfeasible given the inertia in our energy system (e.g. the number of years it takes to turn over a fleet of vehicles or retrofit a power plant). Later peak years could require unprecedented rates of decline.

And even with strong international climate policies, more rapid decarbonization (the rate of decrease in emissions per unit of GDP) will require higher costs and major policy change. In order to avoid high rates of decarbonization, milestones along an emissions pathway (e.g. 2020, 2030, 2050 targets) can be very helpful, especially for the investment community. Milestones will also ensure that an unfair burden is not placed on the next generation.

But perhaps most importantly, one can argue for an even smaller budget and additional emissions constraints because non-CO2 gases are not included in 1 trillion tonne C figure. For example, short-lived greenhouse gases, such as methane, are not included in – nor necessarily appropriate for4 – the 1 trillion tonne C budget approach because they play a secondary role in influencing long-term warming.

However, when non-CO2 forcings are taken into account, the budget is reduced and that budget may depend on the scenario studied. For example, according to one scenario studied in the IPCC AR5 (RCP 2.6), when non-CO2 greenhouse gases are considered, the budget drops much lower to 790 PgC. This scenario would imply a leftover budget of only 275 PgC and an exhaustion of the budget in less than two decades if carbon dioxide emissions increase at a carbon-intensive rate.5

  1. Sustained negative emissions past the exhaustion date could theoretically compensate for positive emissions, so this analysis is illustrative. ↩︎

  2. See footnote 1 in data table. Assumes RCP 8.5 scenario. ↩︎

  3. Nature, Proceedings of the National Academy of Sciences, Nature, Nature, American Meteorological Society, Nature ↩︎

  4. Short-lived GHGs may be better controlled with a separate approach. For more information see here and here ↩︎

  5. See Table. Budget is exhausted in 2032 if RCP 8.5 CO2 growth rates are assumed. ↩︎