Periodic Reporting for period 4 - Couplet (Transient climate change in the coupled atmosphere--ocean system)
Okres sprawozdawczy: 2023-04-01 do 2024-09-30
Worldwide climate change of growing magnitude is expected during coming decades because of global emissions of carbon dioxide and other gases. The severity of impacts of many aspects of projected change is expected to be greater for larger increase in global mean surface temperature. Although computer climate models have hugely improved over the past several decades, knowledge has grown and confidence increased, it is still very uncertain how much global warming will result for a given amount of extra greenhouse-gas heating.
This uncertain quantity is called the climate sensitivity. Research in recent years has revealed that, as well as being uncertain, the climate sensitivity is not constant, but changes over time as the climate evolves. Moreover, its value is different for each of the various agents that have forced climate change to occur during the past 150 years, which include effects of air pollution and volcanic eruptions as well as CO2 and other greenhouse gases. The inconstancy of climate sensitivity compounds the difficulty of evaluating it from past observations and constraning projections of the future.
The hypothesis of this project is that the variations of the climate sensitivity are related to variations in the geographical patterns of temperature change, which are determined by the responses of both atmosphere and ocean to the forcing agents. The objective of the project is to develop new frameworks for describing and predicting the variations of the coupled atmosphere--ocean climate system, taking into account the influences on and the effects of the geographical patterns. Improved scientific understanding will enable more precise projections for given emissions scenarios. National and international plans for adaptation to and mitigation of climate change depend on such information.
This uncertain quantity is called the climate sensitivity. Research in recent years has revealed that, as well as being uncertain, the climate sensitivity is not constant, but changes over time as the climate evolves. Moreover, its value is different for each of the various agents that have forced climate change to occur during the past 150 years, which include effects of air pollution and volcanic eruptions as well as CO2 and other greenhouse gases. The inconstancy of climate sensitivity compounds the difficulty of evaluating it from past observations and constraning projections of the future.
The hypothesis of this project is that the variations of the climate sensitivity are related to variations in the geographical patterns of temperature change, which are determined by the responses of both atmosphere and ocean to the forcing agents. The objective of the project is to develop new frameworks for describing and predicting the variations of the coupled atmosphere--ocean climate system, taking into account the influences on and the effects of the geographical patterns. Improved scientific understanding will enable more precise projections for given emissions scenarios. National and international plans for adaptation to and mitigation of climate change depend on such information.
In this project,
* We have demonstrated that the climate sensitivity varied over the last 100 years by a factor of two on multidecadal timescales both in reality and in climate model simulations, but the models are quite unrealistic in the timing of the variation. In the models, sensitivity reached its maximum in recent decades, whereas in same period it was at its minimum in the real world, due to an unusual pattern of temperature trends in the Pacific Ocean.
* We have found that this pattern is produced as a response to explosive volcanic eruptions in reality, but not in models. The reason for this difference remains to be found, but it suggests a systematic deficiency of models. (Explosive volcanic eruptions, such as that of Mount Pinatubo in 1991, inject small particles, called aerosol, into the stratosphere. These particles reflect sunshine and thus cause a global cooling for a couple of years, until they have dissipated.)
* We have examined how the climate sensitivity increases as time passes under constant elevated CO2 because of the way in which the pattern of surface temperature change evolves, and we have shown that it is greater for higher CO2 concentration and larger global-mean temperature change. We have produced evidence than the climate sensitivity to CO2 differs among models partly because they predict different patterns of surface temperature change.
* We have developed an improved description of the global energy balance, in which the climate sensitivity depends on the change in the vertical profile of atmospheric temperature. This in turn depends on the geographical pattern of surface temperature change. Greenhouse gases, anthropogenic aerosol produced by pollution, and volcanic stratospheric aerosol all cause different patterns of surface temperature change. Our new theory explains why climate sensitivity is different for these three and other kinds of forcing.
* We have achieved a new understanding of the pathways along which heat is absorbed by the ocean in models as the climate becomes warmer, recognising that different processes dominate at low and high latitude. We have discovered that heat is absorbed at high latitude more readily by the ocean in those models where the density of seawater increases more slowly with increasing depth below the surface. Moreover, in such models the Atlantic meridional overturning circulation (AMOC) is generally stronger. This explains a previously discovered correlation between ocean heat uptake efficiency and the AMOC strength. Using a refined method, we have made a new estimate of rate at which the ocean has warmed since the late 19th century, based on observations.
All these outcomes are consistent with the hypothesis that variations in climate sensitivity can be related to the geographical pattern of climate change, and show that the pattern interacts also with the vertical profile of atmospheric warming and the efficiency of ocean heat uptake. They provide answers to some existing questions, and raise some new ones. Following the conclusion of the project, we will continue to work on applying this new knowledge for refinement of climate projections.
* We have demonstrated that the climate sensitivity varied over the last 100 years by a factor of two on multidecadal timescales both in reality and in climate model simulations, but the models are quite unrealistic in the timing of the variation. In the models, sensitivity reached its maximum in recent decades, whereas in same period it was at its minimum in the real world, due to an unusual pattern of temperature trends in the Pacific Ocean.
* We have found that this pattern is produced as a response to explosive volcanic eruptions in reality, but not in models. The reason for this difference remains to be found, but it suggests a systematic deficiency of models. (Explosive volcanic eruptions, such as that of Mount Pinatubo in 1991, inject small particles, called aerosol, into the stratosphere. These particles reflect sunshine and thus cause a global cooling for a couple of years, until they have dissipated.)
* We have examined how the climate sensitivity increases as time passes under constant elevated CO2 because of the way in which the pattern of surface temperature change evolves, and we have shown that it is greater for higher CO2 concentration and larger global-mean temperature change. We have produced evidence than the climate sensitivity to CO2 differs among models partly because they predict different patterns of surface temperature change.
* We have developed an improved description of the global energy balance, in which the climate sensitivity depends on the change in the vertical profile of atmospheric temperature. This in turn depends on the geographical pattern of surface temperature change. Greenhouse gases, anthropogenic aerosol produced by pollution, and volcanic stratospheric aerosol all cause different patterns of surface temperature change. Our new theory explains why climate sensitivity is different for these three and other kinds of forcing.
* We have achieved a new understanding of the pathways along which heat is absorbed by the ocean in models as the climate becomes warmer, recognising that different processes dominate at low and high latitude. We have discovered that heat is absorbed at high latitude more readily by the ocean in those models where the density of seawater increases more slowly with increasing depth below the surface. Moreover, in such models the Atlantic meridional overturning circulation (AMOC) is generally stronger. This explains a previously discovered correlation between ocean heat uptake efficiency and the AMOC strength. Using a refined method, we have made a new estimate of rate at which the ocean has warmed since the late 19th century, based on observations.
All these outcomes are consistent with the hypothesis that variations in climate sensitivity can be related to the geographical pattern of climate change, and show that the pattern interacts also with the vertical profile of atmospheric warming and the efficiency of ocean heat uptake. They provide answers to some existing questions, and raise some new ones. Following the conclusion of the project, we will continue to work on applying this new knowledge for refinement of climate projections.
From climate model experiments, we have made various discoveries that were unexpected and advance the state of knowledge substantially:
* That the climate sensitivity varies over time during the last 150 years because of the varying importance of different drivers of change, especially greenhouse gases and volcanic eruptions.
* That there are physical relationships among the climate sensitivity, the change in the vertical profile of atmospheric temperature, and the geographical pattern of surface temperature change.
* That there are physical relationships among the uptake of heat by the ocean at high latitude (especially in the Southern Ocean), the vertical profile of seawater density (especially in mid-latitudes) and the strength of the Atlantic meridional overturning circulation (which is especially influential in the North Atlantic).
We have formulated new quantatitive formulae for climate sensitivity and ocean heat uptake efficiency based on these discoveries.
In addition, we have made a new reconstruction of historical ocean heat uptake on the basis of observed surface temperature change, using an existing method which we substantially improved.
* That the climate sensitivity varies over time during the last 150 years because of the varying importance of different drivers of change, especially greenhouse gases and volcanic eruptions.
* That there are physical relationships among the climate sensitivity, the change in the vertical profile of atmospheric temperature, and the geographical pattern of surface temperature change.
* That there are physical relationships among the uptake of heat by the ocean at high latitude (especially in the Southern Ocean), the vertical profile of seawater density (especially in mid-latitudes) and the strength of the Atlantic meridional overturning circulation (which is especially influential in the North Atlantic).
We have formulated new quantatitive formulae for climate sensitivity and ocean heat uptake efficiency based on these discoveries.
In addition, we have made a new reconstruction of historical ocean heat uptake on the basis of observed surface temperature change, using an existing method which we substantially improved.