Richard Betts | University of Exeter (original) (raw)

Papers by Richard Betts

Earth System Dynamics, 2012

The role of global vegetation on the large-scale tropical circulation is examined in the version ... more The role of global vegetation on the large-scale tropical circulation is examined in the version 3 Hadley Centre climate model (HadCM3). Alternative representations of global vegetation cover from observations and a dynamic global vegetation model (DGVM) were used as the landcover component for a number of HadCM3 experiments under a nominal present day climate state, and compared to the simulations using the standard land cover map of HadCM3. The alternative vegetation covers result in a large scale cooling of the Northern Hemisphere extra-tropics relative to the HadCM3 standard, resulting in a southward shift in the location of the inter-tropical convergence zone (ITCZ). A significant reduction in Indian monsoon precipitation is also found, which is related to a weakening of the South Asian monsoon circulation, broadly consistent with documented mechanisms relating to temperature and snow perturbations in the Northern Hemisphere extra-tropics in winter and spring, delaying the onset of the monsoon. The role of the Northern Hemisphere extra-tropics on tropical climate is demonstrated, with an additional representation of vegetation cover based on DGVM simulated changes in Northern Hemisphere vegetation from the end of the 21st Century. This experiment shows that through similar processes the simulated extra-tropical vegetation changes in the future contribute to a strengthening of the South Asian monsoon in this model. These findings provide renewed motivation to give careful consideration to the role of global scale vegetation feedbacks when looking at climate change, and its impact on the tropical circulation and South Asian monsoon in the latest generation of Earth System models.

Journal of Climate, 2006

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between c... more Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100...

The RF represents the stratospherically adjusted radiative fl ux change evaluated at the tropopau... more The RF represents the stratospherically adjusted radiative fl ux change evaluated at the tropopause, as defi ned in the TAR. Positive RFs lead to a global mean surface warming and negative RFs to a global mean surface cooling. Radiative forcing, however, is not designed as an indicator of the detailed aspects of climate response. Unless otherwise mentioned, RF here refers to global mean RF. Radiative forcings are calculated in various ways depending on the agent: from changes in emissions and/or changes in concentrations, and from observations and other knowledge of climate change drivers. In this report, the RF value for each agent is reported as the difference in RF, unless otherwise mentioned, between the present day (approximately 2005) and the beginning of the industrial era (approximately 1750), and is given in units of W m-2. 2 90% confi dence ranges are given in square brackets. Where the 90% confi dence range is asymmetric about a best estimate, it is given in the form A [-X, +Y] where the lower limit of the range is (A-X) and the upper limit is (A + Y). 3 The use of 'extremely likely' is an example of the calibrated language used in this document, it represents a 95% confi dence level or higher; 'likely' (66%) is another example (See Box TS.1). 4 Estimates of RF are accompanied by both an uncertainty range (value uncertainty) and a level of scientifi c understanding (structural uncertainty). The value uncertainties represent the 5 to 95% (90%) confi dence range, and are based on available published studies; the level of scientifi c understanding is a subjective measure of structural uncertainty and represents how well understood the underlying processes are. Climate change agents with a high level of scientifi c understanding are expected to have an RF that falls within their respective uncertainty ranges (See Section 2.9.1 and Box TS.1 for more information). FAQ 2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. (Data combined and simplifi ed from Chapters 6 and 2 of this report.) (continued) Changes in Atmospheric Constituents and in Radiative Forcing Chapter 2 Figure 2.3. Recent CO 2 concentrations and emissions. (a) CO 2 concentrations (monthly averages) measured by continuous analysers over the period

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010

The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a ra... more The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a range of scenarios of future greenhouse-gas emissions without policies to specifically reduce emissions, and concluded that these would lead to an increase in global mean temperatures of between 1.6°C and 6.9°C by the end of the twenty-first century, relative to pre-industrial. While much political attention is focused on the potential for global warming of 2°C relative to pre-industrial, the AR4 projections clearly suggest that much greater levels of warming are possible by the end of the twenty-first century in the absence of mitigation. The centre of the range of AR4-projected global warming was approximately 4°C. The higher end of the projected warming was associated with the higher emissions scenarios and models, which included stronger carbon-cycle feedbacks. The highest emissions scenario considered in the AR4 (scenario A1FI) was not examined with complex general circulation models ...

Nature Geoscience, 2013

How tropical forest carbon stocks might alter in response to changes in climate and atmospheric c... more How tropical forest carbon stocks might alter in response to changes in climate and atmospheric composition is uncertain.
However, assessing potential future carbon loss from tropical forests is important for evaluating the efficacy of programmes for reducing emissions from deforestation and degradation.
Uncertainties are associated with different carbon stock responses in models with different representations of vegetation processes on the one hand1–3, and differences in projected changes in temperature and precipitation patterns on the other hand4,5. Here we present a systematic exploration of these sources of uncertainty, along with uncertainty arising from different emissions scenarios for all three main tropical forest regions: the Americas (that is, Amazonia and Central America), Africa and Asia. Using simulations with 22 climate models and the MOSES–TRIFFID land surface scheme, we find that only in one5 of the simulations are tropical forests projected to lose biomass by the end of the twenty-first century—and then only for the Americas. When comparing with alternative models of plant physiological processes1,2, we find that the largest uncertainties are associated with plant physiological responses, and then with future emissions scenarios. Uncertainties from differences in the climate projections are significantly smaller.
Despite the considerable uncertainties, we conclude that there is evidence of forest resilience for all three regions.

Earth System Dynamics, 2012

The role of global vegetation on the large-scale tropical circulation is examined in the version ... more The role of global vegetation on the large-scale tropical circulation is examined in the version 3 Hadley Centre climate model (HadCM3). Alternative representations of global vegetation cover from observations and a dynamic global vegetation model (DGVM) were used as the landcover component for a number of HadCM3 experiments under a nominal present day climate state, and compared to the simulations using the standard land cover map of HadCM3. The alternative vegetation covers result in a large scale cooling of the Northern Hemisphere extra-tropics relative to the HadCM3 standard, resulting in a southward shift in the location of the inter-tropical convergence zone (ITCZ). A significant reduction in Indian monsoon precipitation is also found, which is related to a weakening of the South Asian monsoon circulation, broadly consistent with documented mechanisms relating to temperature and snow perturbations in the Northern Hemisphere extra-tropics in winter and spring, delaying the onset of the monsoon. The role of the Northern Hemisphere extra-tropics on tropical climate is demonstrated, with an additional representation of vegetation cover based on DGVM simulated changes in Northern Hemisphere vegetation from the end of the 21st Century. This experiment shows that through similar processes the simulated extra-tropical vegetation changes in the future contribute to a strengthening of the South Asian monsoon in this model. These findings provide renewed motivation to give careful consideration to the role of global scale vegetation feedbacks when looking at climate change, and its impact on the tropical circulation and South Asian monsoon in the latest generation of Earth System models.

Journal of Climate, 2006

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between c... more Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100...

The RF represents the stratospherically adjusted radiative fl ux change evaluated at the tropopau... more The RF represents the stratospherically adjusted radiative fl ux change evaluated at the tropopause, as defi ned in the TAR. Positive RFs lead to a global mean surface warming and negative RFs to a global mean surface cooling. Radiative forcing, however, is not designed as an indicator of the detailed aspects of climate response. Unless otherwise mentioned, RF here refers to global mean RF. Radiative forcings are calculated in various ways depending on the agent: from changes in emissions and/or changes in concentrations, and from observations and other knowledge of climate change drivers. In this report, the RF value for each agent is reported as the difference in RF, unless otherwise mentioned, between the present day (approximately 2005) and the beginning of the industrial era (approximately 1750), and is given in units of W m-2. 2 90% confi dence ranges are given in square brackets. Where the 90% confi dence range is asymmetric about a best estimate, it is given in the form A [-X, +Y] where the lower limit of the range is (A-X) and the upper limit is (A + Y). 3 The use of 'extremely likely' is an example of the calibrated language used in this document, it represents a 95% confi dence level or higher; 'likely' (66%) is another example (See Box TS.1). 4 Estimates of RF are accompanied by both an uncertainty range (value uncertainty) and a level of scientifi c understanding (structural uncertainty). The value uncertainties represent the 5 to 95% (90%) confi dence range, and are based on available published studies; the level of scientifi c understanding is a subjective measure of structural uncertainty and represents how well understood the underlying processes are. Climate change agents with a high level of scientifi c understanding are expected to have an RF that falls within their respective uncertainty ranges (See Section 2.9.1 and Box TS.1 for more information). FAQ 2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. (Data combined and simplifi ed from Chapters 6 and 2 of this report.) (continued) Changes in Atmospheric Constituents and in Radiative Forcing Chapter 2 Figure 2.3. Recent CO 2 concentrations and emissions. (a) CO 2 concentrations (monthly averages) measured by continuous analysers over the period

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010

The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a ra... more The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a range of scenarios of future greenhouse-gas emissions without policies to specifically reduce emissions, and concluded that these would lead to an increase in global mean temperatures of between 1.6°C and 6.9°C by the end of the twenty-first century, relative to pre-industrial. While much political attention is focused on the potential for global warming of 2°C relative to pre-industrial, the AR4 projections clearly suggest that much greater levels of warming are possible by the end of the twenty-first century in the absence of mitigation. The centre of the range of AR4-projected global warming was approximately 4°C. The higher end of the projected warming was associated with the higher emissions scenarios and models, which included stronger carbon-cycle feedbacks. The highest emissions scenario considered in the AR4 (scenario A1FI) was not examined with complex general circulation models ...

Nature Geoscience, 2013

How tropical forest carbon stocks might alter in response to changes in climate and atmospheric c... more How tropical forest carbon stocks might alter in response to changes in climate and atmospheric composition is uncertain.
However, assessing potential future carbon loss from tropical forests is important for evaluating the efficacy of programmes for reducing emissions from deforestation and degradation.
Uncertainties are associated with different carbon stock responses in models with different representations of vegetation processes on the one hand1–3, and differences in projected changes in temperature and precipitation patterns on the other hand4,5. Here we present a systematic exploration of these sources of uncertainty, along with uncertainty arising from different emissions scenarios for all three main tropical forest regions: the Americas (that is, Amazonia and Central America), Africa and Asia. Using simulations with 22 climate models and the MOSES–TRIFFID land surface scheme, we find that only in one5 of the simulations are tropical forests projected to lose biomass by the end of the twenty-first century—and then only for the Americas. When comparing with alternative models of plant physiological processes1,2, we find that the largest uncertainties are associated with plant physiological responses, and then with future emissions scenarios. Uncertainties from differences in the climate projections are significantly smaller.
Despite the considerable uncertainties, we conclude that there is evidence of forest resilience for all three regions.