Climate Change Canada

 

3.2: Future climate change and climate forcing

Emissions of greenhouse gases from human activity, particularly carbon dioxide, will largely determine the magnitude of climate change over the next century. Therefore, reducing human emissions will reduce future climate change.

Projections of future climate change require projections of future climate forcing — the external drivers of change such as GHGs and aerosols. These projections, in turn, arise from scenarios of future GHG and aerosol emissions, which are based on varying assumptions about how human activities, such as fossil fuel consumption and land use, will change. Future emission scenarios are typically developed using integrated assessment models, which combine economic, demographic, and policy modelling with simplified physical climate models to make projections of population growth, economic development, land use, and the implications of different policy options on climate-relevant emissions. As there is large uncertainty in the social and economic aspects of such projections, a range of scenarios is generally provided, ranging from those in which emissions are aggressively reduced to those with limited actions taken to mitigate emissions.

The projections described in the IPCC Fifth Assessment (see Chapter 1) were based on a suite of future forcing scenarios called Representative Concentration Pathways (RCPs) that cover the period from 2006 onward (van Vuuren et al., 2011). The RCPs are identified by a number indicating the change in radiative forcing — the imbalance between the solar radiation entering the climate system and the infrared (longwave) radiation leaving it caused by greenhouse gases and other external drivers (see Chapter 2, Section 2.3.1) — by the end of the 21st century. RCP2.6 represents a low emission pathway with a change in radiative forcing of roughly 2.6 W/m2, RCP4.5 and RCP6 represent intermediate emission pathways, and RCP8.5 represents a pathway with continued growth in GHG emissions, leading to a radiative forcing of roughly 8.5 W/m2 at the end of the century. In this report, we will refer to climate scenarios based on RCP2.6 as “low emission scenarios,” those based on RCP4.5 and RCP6 as “medium emission scenarios,” and those based on RCP8.5 as “high emission scenarios.” For each RCP, the integrated assessment models provide a comprehensive time series of emissions and concentrations of individual GHGs (CO2, methane [CH4], nitrous oxide [N2O], chlorofluorocarbons, etc.), along with aerosol emissions and land-use change. All of these forcings are inputs to Earth system models, which then simulate the future response of the climate system, including biogeochemical feedbacks, to these external forcing scenarios.

The RCPs supersede the so-called “SRES scenarios” (Nakicenovic et al., 2000), which served as the basis for model runs reported in the IPCC Fourth Assessment Report. Although there are differences in detail, the SRES A2 forcing scenario is roughly comparable to RCP8.5, the SRES A1B scenario is roughly midway between RCP6 and RCP8.5, and SRES B1 is roughly comparable to RCP4.5 (e.g., Burkett et al., 2014). There was no SRES forcing scenario comparable to RCP2.6. These forcing scenarios are updated every few years, and new shared socioeconomic pathways (building upon the RCPs) will be used in model runs that will feed into the upcoming IPCC Sixth Assessment (e.g., Riahi et al., 2017).

In all cases, CO2 is the largest contributor to the historical and projected change radiative forcing, followed by CH4 and N2O (Myhre et al., 2013; Collins et al., 2013). This means that future changes in human emissions of CO2 will largely determine future climate.

An illustration of aspects of the RCPs is shown in Figure 3.1. It is important to note that no likelihoods are ascribed to these future forcing scenarios — they are all deemed plausible, although, as emissions continue to increase, low emission pathways become more difficult to achieve (e.g., Millar et al., 2017). The spread across the RCPs represents some measure of our uncertainty about how socioeconomic factors may change in the future, especially how aggressively humans will pursue emission mitigation, and therefore the pace at which humans will continue to drive climate change. The low emission (RCP2.6) scenario is consistent with limiting global temperature increase to roughly 2ºC above the pre-industrial value (see Section 3.3.3) and is therefore roughly compatible with the global temperature goal agreed to in the Paris Agreement (UNFCCC, 2015). This scenario requires global CO2 emissions to peak almost immediately and reduce to near zero well before the end of the century. Global annual CO2 emissions reached about 10 Gt of carbon (about 37 GtCO2) in 2017 (Le Quéré et al., 2017)

 

 

3.3: Modelling the response of the climate system to external forcing

In the near term (to approximately 2040), projected warming will be similar under all emission pathways. But by the late 21st century, the differences between possible emissions pathways will have a considerable effect. Available estimates indicate that the global climate will warm by an additional 1ºC (for a low emission scenario) to 3.7ºC (for a high emission scenario). Scenarios that would limit warming to an additional 1ºC or less require rapid and deep emission reductions.

 

3.3.1: Earth system models

Earth system models are based on a mathematical representation of the behaviour of the atmosphere, ocean, land surface, and cryosphere. They simulate a virtual planet using powerful supercomputers, allowing scientists to probe the connections between various physical and biogeochemical processes, e.g., how the ocean takes up heat and carbon, stores and then redistributes it. Two main ways such models are used are (1) to compare simulations with and without historical forcings to determine human versus natural forcing effects, and (2) to simulate future climate in response to various forcing scenarios.

Earth system models have some features in common with global weather-prediction models (used to make daily weather forecasts) but do not depend on the use of observations as inputs and typically operate at somewhat lower spatial resolution (i.e., the level of spatial detail is often limited to features with scales of a hundred kilometres or larger). The lower resolution is necessitated by the computing demand of the long simulations that are required. Simulations begin with the historical period (from 1850 to present), driven by observationally based climate forcing (e.g., historical changes in GHG concentrations), and then continue into the future, using different forcing scenarios (such as those described in the previous section) out to year 2100 or further.

Earth system models represent an evolution from earlier physical climate models (representing the coupled atmosphere, ocean, land, and sea ice components of the climate system) to models that go beyond this with explicit representation of the carbon cycle (Flato, 2011; Flato et al., 2013). Including carbon and other biogeochemical cycles in the models allows simulation of global interactions among ecosystems, carbon, and climate, as well as several terrestrial processes that occur at high latitudes. Changes to snow and sea ice can cause positive (amplifying) snow/ice albedo feedbacks in the climate system (Euskirchen et al., 2016; Kashiwase et al., 2017; see Chapter 2, Box 2.4). As temperatures increase, the spatial extent of snow and sea ice declines, reducing the reflectivity of land and oceans, allowing more solar radiation to be absorbed, and hence further increasing temperatures. This feedback makes an important contribution to the higher rate of warming in the Arctic region, called Arctic amplification (FAQ 3.1; see Section 3.3.3). The models can also simulate increased growth of vegetation at northern high latitudes in response to a warming climate, an effect that may reduce the land surface albedo and affect the exchange of energy and water between the land and the atmosphere (Forkel et al., 2016). Changes in permafrost in response to changing climate — leading to changes in hydrological conditions and CH4 release (Schuur et al., 2008) — are also now included in some models.

How do we know that models are accurately projecting future climate? One method of measuring whether models can realistically represent the complex interconnections among climate processes is to gauge their ability to reproduce past changes. Simulations using observationally based historical forcing from 1850 onward provide the opportunity to directly compare model results to observations. The IPCC Assessment Reports have traditionally included a chapter devoted to this type of model evaluation (e.g., Flato et al., 2013), and these provide a synthesis of the large number of scientific papers on model performance. Figure 3.5 provides one example of model evaluation, comparing the annual global mean surface air temperature from several different sources with simulations from 36 different models that participated in the fifth phase of the Coupled Model Intercomparison Project (CMIP5; see Box 3.1).  Earth system models are able to reproduce the observed long-term increase in temperature (heavy black lines), along with the sporadic cooling that follows large explosive volcanic eruptions. The magnitude of natural year-to-year variability is also well simulated (thin lines), although one does not expect individual ups and downs to coincide (as each model simulates its own internal variability). The heavy red line in the figure shows the multi-model average, which is an approximation of the response of the climate system to external forcing (changing GHG concentrations and aerosol amount, land-use change, variations in solar irradiance, and volcanic aerosol), upon which internal variability is superimposed. The difference between observed temperature and the multi-model average from roughly 2000 onward has been extensively analyzed (e.g., Fyfe et al., 2016) and is due to a combination of small errors in the observational record, decadal timescale internal variability, and incomplete early 21st century volcanic forcing in the models.