Coupled Climate Models

Most GCMs neglect the feedback between climate and the biosphere and the complex carbon-cycle feedbacks that are involved. As Randall et al., (2007) states, however, “coupled climate models perform generally better than atmosphere-only models, and reveal the amplifying roles of ocean and land surface feedbacks in climate change”. The ocean is as important a feature of the climate models

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as the atmosphere, since ocean currents transport vast amounts of heat from the equator to the poles, as well as having a very large thermal inertia (Lovejoy and Hannah, 2005). Also, the exchange of heat, momentum and water vapour between the ocean and atmosphere plays a part in regulating the climate. Representation of such features in coupled atmosphere-ocean models has, only in the last decade or so, been used to portray an overall picture of the climate for the future.

Coupled climate models are an essential tool, for both making predictions, and for increasing the understanding of feedbacks and sensitivities. Only about half of the current emissions of carbon dioxide are absorbed by oceans and ecosystems, but the facilitation of such processes is climate dependant. The future atmospheric CO2 concentration is therefore not easy to predict in relation to working out future climate change (Dufresne et al., 2002). The rising concentrations of the gas is believed to cause an increase in plant photosynthesis (DeLucia et al., 1999) and carbon dissolution in water (Oeschger et al., 1975), whereas the related climate change associated with rising CO2

emissions is known to reduce ocean carbon uptake (Sarmiento et al., 1998) and the terrestrial carbon uptake (Cramer et al., 2001), resulting in a positive feedback.

The inclusion of the terrestrial biosphere model, that replicate changes in terrestrial carbon sources and sinks, into fully coupled climate models, is a leading development in climate science, resulting in new and potentially important feedback into the simulated climate system on time scales of decades to centuries. Some coupled climate/carbon-cycle models are also linked to a dynamic global vegetation model (DGVM), so as well as the exchange of CO2 between the atmosphere and the ocean, and the relationship between soil carbon and atmospheric levels of CO2, the dynamics and extent of up to five functional types of plant within grid boxes of the models are also considered. As Cox et al. (2000) infers, this allows the interplay between factors of the climate system and external forcings to be fully represented.

2.7.1 Coupled Carbon-cycle/Climate Model Studies

The first two studies to investigate the effect of incorporating an interactive carbon-cycle into an AOGCM discovered that there was a positive feedback response, with an acceleration of global warming being the outcome (Friedlingstein et al., 2006). Cox et al. (2000) used a fully coupled, three dimensional carbon-climate model – the HadCM3, which was also coupled to an ocean carbon-cycle model (HadOCC) and a DGVM, with the atmospheric physics and dynamics consistent with the HadCM3. Emission scenario IS92a (from the first set of IPCC scenarios) was used for the transient simulations between 1860-2100, reproducing observational records and thus lending the model

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validity. Sulphate aerosols, however, and their radiative effects were neglected, a possible flaw due to the cooling effect created when these aerosols are formed. Three varying simulations were run to demonstrate the effect of climate-carbon cycle feedbacks. Dufresne et al. (2002) used an AOGCM - Institute Pierre Simon Laplace model IPSL-CM2, coupled to land and ocean carbon models to simulate the progression of climate and atmospheric CO2 between 1860-2100; a control simulation was run with no anthropogenic CO2 sources. The simulation used IPCC SRES98-A2 emission scenario from 1990 to 2100 (Nakicenovic et al., 2000), which is an updated version of the earlier IPCC IS92a scenario. Both studies used similar methodologies, although Dufresne et al. (2002) applied no restoring term of flux adjustments and they did not use a terrestrial model that accounts for vegetation dynamics.

In Cox et al. (2000) simulations it was predicted that, as emissions of CO2 increase and thus atmospheric levels increase, by 2050 the land biosphere becomes a strong source of CO2 rather than a sink, with ‘widespread climate-driven loss of soil carbon’. Normally the rates of photosynthesis as well as terrestrial carbon stocks increase when there is an increase in atmospheric CO2, as Hughes (2000) discusses, with CO2 having a direct fertilisation effect on plants. However, with the indirect effect of CO2 being warming, other factors like plant maintenance and soil respiration consequently increase and thus reverse the initial increase in terrestrial carbon storage levels, leading to the land biosphere becoming a source of CO2 (Cox et al., 2000). This becomes more apparent when temperatures increase dramatically midway through the 21^st century, with a reduction of terrestrial carbon by about 170GtC between 2000 and 2100, and a consequent increase in the rate of CO2 in the atmosphere (Cox et al., 2000). Figure 2.9 shows that the terrestrial biosphere takes up CO2 at a decreasing rate from 2010 onwards, becoming a net source at around 2050. By 2100 this source from the land almost balances the oceanic sink, so that atmospheric carbon content is increasing at about the same rate as the integrated emissions (Cox et al., 2000). Both Dufrense et al. (2002) and Cox et al. (2000) forecast that the CO2 induced climate change will reduce the land carbon uptake, with a bigger percent of anthropogenic CO2 left in the atmosphere.

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Figure 2.9 Budgets of carbon during the fully coupled simulation (Cox et al., 2000).

The thick line shows the simulated change in atmospheric CO2. The thinner line shows the integrated impact of the emissions, and of land and ocean fluxes, on the atmospheric CO2 increase with negative values implying net uptake of CO2 (Cox et al., 2000).

In the experiment reported by Cox et al. (2000) concentrations of carbon in the model reach around 980 ppmv (parts per million by volume) by 2100, compared to about 730 ppmv calculated using the IS92a scenario. As a result the global-mean land temperature increases by about 8 K between 1860 and 2100, with the rise being only 5.5 K in the standard non-coupled scenario, exemplifying the difference a coupled climate model can make (Cox et al., 2000). In the Dufresne et al. (2002) scenario simulation, it was found that by 2100 the atmospheric CO2 concentration is 770ppmv, with a global temperature increase of 3°C (4.4°C over the continents) and a small precipitation increase (4%). The oceanic circulation shows a small but significant reduction of the thermohaline circulation, and of the deep convection at high latitudes. The warming in the Hadley model may be larger due to the fact that Cox et al. (2000) account for more than just CO2 emissions (CH3, N20 etc), whereas IPSL only accounts for CO2. It is not due to differences in the IPCC forcing scenarios, but rather to large differences in the model sensitivities (Friedlingstein et al., 2003).

In Cox et al. (2000), although the oceanic CO2 uptake over the 21^st Century does increase overall, the efficiency of the uptake decreases to some extent because of the ‘nonlinear dependence of the partial pressure of dissolved CO2 on the total ocean carbon concentration’. As Sarmiento et al.

(1998) discuss, climate change may be a contributing factor to this reduction, as the consequent 2050

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warming and increased stratification on the ocean has an effect on the downward flux of carbon, with solubility of CO2 decreasing relative to a constant climate control scenario. When Sarmiento et al. (1998) used an AOGCM (simulating the observed temperature record) coupled to an ocean carbon only model, they found that climate change reduces ocean carbon uptake, which contradicts with the findings of both Dufresne et al. (2002), and Cox et al. (2000). This indicates that atmosphere and ocean carbon models may be more reliable; they find that reduced land uptake of CO2, through enhanced atmospheric CO2 results in increased ocean uptake, a feedback that must be neglected in ocean carbon only studies (Dufresne et al., 2002). It is important to consider the feedbacks between the climate system and the global carbon cycle simultaneously.

Reasons for reduced land CO2 uptake over time are due to reductions in net primary production (NPP) for both models with the associated warming and soil drying, but the soil respiration rate (SRR) increases much more in the Cox et al. (2000) model due to warming, compared with Dufresne et al.

(2002). Change in landcover due to deforestation is not accounted for in either model, which could indicate the models overestimate CO2 absorption in regions where deforestation may occur in the future. The effect of temperature on soil respiration is still a debateable topic which puts uncertainty on all results calculated. Changes in landcover and land use, through human intervention, will also play a role in terrestrial carbon uptake and whether it acts as a source rather than a sink over the 21^st century (Cox et al., 2000).

Both studies confirm that there is positive feedback between the climate and carbon cycle, as a result of climate impact on the terrestrial biosphere, but that it is the land response to global warming that essentially explains the differences between the Dufresne IPSL model and the Cox Hadley model results, and thus more research into model sensitivities is required. Being the first two studies to investigate the effect of a carbon-cycle incorporated into a climate model, the studies have shown the importance of the climate-carbon cycle, with results showing a substantial increase in temperature when compared to standard non-coupled scenarios. If predictions for future climate change are to be successfully calculated such models need to be utilised.

The ocean plays a vital part in the climate system, distributing heat and acting as a CO2 sink. The next section describes how the global ocean circulation system can have a strong effect on global climate change, and how some models have shown that there may not be an increase in temperature in years to come, particularly over Europe.

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