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.
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