Climate Change Canada

 

3.4: Cumulative carbon dioxide and global temperature change

Global temperature change is effectively irreversible on multi-century timescales. This is because the total amount of carbon dioxide emitted over time is the main determinant of global temperature change and because carbon dioxide has a long (century-scale) lifetime in the atmosphere.

CO2 is the largest contributor to anthropogenic radiative forcing and hence the dominant driver of anthropogenic climate change (Myhre et al., 2013) (see Chapter 2, Section 2.3.2). It also has a very long atmospheric lifetime (see Box 3.3). These properties mean that CO2 emissions are the dominant control on future climate change. Traditionally, the focus has been on annual average emissions and their changes over time. However, recent research has found that the accumulation of CO2 emissions over time are what determine global warming. From this research has emerged the concept of a level of cumulative emissions (called a cumulative carbon emissions budget) that must not be exceeded in order to limit temperature increases to a certain threshold.

 

3.4.1: The climate response to cumulative carbon dioxide emissions

The IPCC Fifth Assessment Report found that warming induced by CO2 at any point in time since the beginning of the Industrial Era is proportional to the total amount of CO2 emitted up to that time (cumulative CO2 emissions; IPCC, 2013). This relationship has been seen in a range of climate models, across a range of emissions pathways, and even at high levels of cumulative emissions (Tokarska et al., 2016). Figure 3.8 shows that average warming is closely proportional to cumulative CO2 emissions for the CMIP5 models’ simulation of a CO2 increase of 1% per year (thin black line). In this idealized simulation, atmospheric CO2 concentration increases from its 1850 value of around 285 ppm by 1% per year until its concentration quadruples in 140 years to about 1140 ppm. The relationship between cumulative emissions of CO2 and global mean surface temperature (GMST) is altered somewhat by the effects of other climate forcing agents (such as CH4, N2O, and various aerosols) that are included in the RCP scenarios, as shown by the divergence of the colored lines in Figure 3.8 from the thin black CO2-only line. Yet the total warming (due to CO2 and other climate forcing agents) is approximately the same, as a function of cumulative emissions, across the four RCP scenarios shown in Figure 3.8. There is uncertainty in the relationship between warming and cumulative emissions, indicated by the shaded bands in the figure, and this must be taken into account when interpreting the results.

This relationship between cumulative CO2 emissions and the increase in GMST can be used to estimate the maximum amount of CO2 that can be emitted while limiting the temperature increase to a certain level. So, for example, in order to limit global warming to less than 2ºC, as agreed in the Paris Agreement (UNFCCC, 2015), cumulative emissions of CO2 must stay below a given level. Because of the uncertainty in this relationship, a likelihood must be attached to this level. Hence, the IPCC (2013) assessed that, to have a 50% chance of keeping global warming to less than 2ºC, CO2 emissions from 2011 onward would have to remain below 1300 billion tonnes of CO2 (GtCO2), roughly equal to what has already been emitted since the beginning of the Industrial Era. For a 50% chance of keeping the temperature increase to less than 1.5ºC, emissions from 2011 onward would have to be limited to 550 GtCO2. Similar carbon emissions budgets were obtained using an integrated assessment model driven by a broader range of scenarios, an approach that may be more robust (Rogelj et al., 2016). The median IPCC (2014) 1.5ºC emissions budget of 550 GtCO2 relative to 2011 is only 13.8 years of CO2 emissions at current levels of approximately 40 Gt CO2 per year, and we have already used about six years of this. However, several recent studies calculated this budget using an alternative approach, based on an estimate of human-caused global warming from pre-industrial times to 2015 of approximately 0.9ºC (e.g., Millar et al., 2017). This leaves room for an additional approximately 0.6ºC of warming to be consistent with a 1.5ºC target. From this, cumulative carbon emissions budgets consistent with limiting warming to 0.6ºC relative to 2010–2019 with 50% or more chance were estimated to be 760–850 GtCO2 (Millar et al., 2017; Goodwin et al., 2018; Tokarska and Gillett, 2018), substantially more than the 390 GtCO2 (from 2015) assessed by IPCC (2014). Conversely, accounting for carbon-cycle feedbacks involving permafrost, which were not included in the models assessed by IPCC (2014), would somewhat increase the warming for a given level of CO2 emissions and hence somewhat reduce the emissions budgets, particularly at higher warming levels (MacDougall et al., 2015). The upcoming IPCC Special Report on Global Warming of 1.5ºC will comprehensively assess these emissions budgets and give an updated estimate of the remaining allowable emissions to meet the global temperature target under the Paris Agreement.

 

3.4.2: Irreversibility of climate change

Earth system model simulations of the response to CO2 emissions show that GMST remains approximately constant for many centuries following a cessation of emissions (Collins et al., 2013). For example, GMST remains high in two simulations of Environment and Climate Change Canada’s first-generation Earth system model, CanESM1, under a scenario in which CO2 emissions increase and subsequently cease, being reduced to zero in 2010 or in 2100 (Figure 3.9; Gillett et al., 2011). Similar results are obtained using other models (e.g., Matsuno et al., 2012; Matthews and Caldeira, 2008; Frölicher and Joos, 2010). Thus, regardless of when emissions cease, GMST remains approximately constant for the subsequent millennium.

Ceasing emissions of aerosols, which are short-lived and that largely exert climate-cooling effects (see Box 3.3) would lead to rapid warming, whereas ceasing short-lived GHG emissions would cause cooling (Collins et al., 2013). The response to a cessation of emissions of other long-lived GHGs is qualitatively similar to that to CO2 (Smith et al., 2012), taking a very long time to reduce temperature. While GMST is expected to remain constant after emissions cease, other aspects of the climate system are expected to continue to change. Vegetation, ice sheet volume, deep ocean temperature, ocean acidity, and sea level are projected to change for centuries after stabilization of GMST (Collins et al., 2013)

 

3.5: Regional downscaling

Climate projections are based on computer models that represent the global climate system at coarse resolution. Understanding the effects of climate change for specific regions benefits from methods to downscale these projections. However, uncertainty in climate projections is larger as one goes from global to regional to local scale.