Clarifying uncertain elements in the prediction ofglobal warming and improving climate models

In the IPCC Third Assessment Report published in 2001, the predicted increase in the Earth's surface temperature in 2100 caused by global warming, estimated by research organizations around the world, ranges between 1.4ºC to 5.8ºC. One of the major factors in this is thought to be that the clouds' feedback effect has not been sufficiently delineated. Dr. Tsushima presented a way to determine the clouds' feedback effect by analyzing their radiative changes together with the shifts in global surface temperatures using observation data from satellites. She then presented issues related to climate models and the need for improvement.

Dr. Yoko Tsushima
Researcher, Global Warming Research Program

What is the clouds' feedback effect?

When we watch the Earth from space, clouds play several important roles in maintaining the radiative balance. The Earth is heated by the absorption of solar rays, and it is cooled by the release of thermal emissions. Clouds reflect sunlight and cool the Earth's surface. They absorb the thermal energy emitted from the Earth's surface, and re-release it at cloud level. Because the temperature is lower at higher altitudes, clouds reduce the energy emitted from the Earth, thereby warming the planet. If global warming advances, these radiative characteristics of the clouds may change, further modifying the magnitude of the Earth's surface temperature*1 increase. This is called the clouds' feedback effect.

Annual variation of the global mean surface temperature-one of the symptoms of global warming

The feedback of clouds due to global warming is inadequately understood, and thus it is not expressed sufficiently in climate models. This is a major reason why the predicted value of the temperature increase published by research organizations in each country varies by as much as 4ºC.

In order to reduce such uncertainty, I have focused on the annual temperature variation of the Earth. The global mean surface temperature increases by 3.3ºC from January to July. This temperature change can be considered to be one of the climate changes. I have thought that the difference from the climate model could be detected by studying cloud feedback from actual observed data and comparing this feedback with climate models. I analyzed the average cloud radiation per unit of time across the entire Earth, data that is collected from a satellite, and estimated cloud radiation feedback against annual variations. My result is that this observed value and the value from climate models differ, and models have some systematic bias.

A clue to improving the climate model was found through the analysis of cloud feedback

Why does cloud feedback show opposing trends between observed data and climate models? In order to understand this, I analyzed the distribution of the amount of global cloud water*2. The reason I focused my study on the amount of cloud water is that a cloud's ability to reflect sunlight depends greatly on this amount and its condition-whether it consists of water or ice. When I analyzed the contribution of cloud water distribution of the model for January and July, I found that the difference in solar cloud feedback mainly comes from the change in cloud water in the temperature band between 0ºC and minus 15ºC. The reason is that the mechanism of the change between a water cloud and an ice cloud has not yet been elucidated. In many of the current climate models, cloud phase is simply categorized: a water cloud is assumed to be above an atmospheric temperature of 0ºC, and an ice cloud is assumed to be below 15ºC.

While insufficiency in the expression of the cloud phase has been pointed out before, I showed for the first time that it actually appears in the difference of cloud feedback between the observation and the models. This is the significance of my study. Looking ahead, we are preparing for the IPCC Fourth Assessment Report and I will continue my research as part of the larger effort to resolve global warming.

  • Clouds' radiative feedback in their annual variations

  • ERBE: Observed value
    CCCR/NIES, MPI, UKMO: Three major climate models used by international research organizations that perform global warming experiments
  • fc (green): Magnitude of cloud feedback combining both solar and thermal radiation, fcs (light blue): Cloud feedback of solar radiation, fcl (red): Cloud feedback of thermal radiation
  • The vertical axis indicates the direction of the effect on the Earth: warming (+) and cooling (-) of the Earth.

  • Reflection of solar rays:
    In observed values, the effect became smaller with the temperature increase, indicating the warming of the Earth.
    All three models showed the opposite result (cooling of the Earth).
  • Regarding the absorption of radiated heat:
    In observed values, its effect on global warming weakened, and the tendency for heat to be released from the Earth strengthened.
    All three models showed the opposite result (warming of the Earth).
*1 Assuming that only the level of CO2 will double in the near future with no other changes in basic global climate conditions, it is said that the average temperature of the Earth's surface will increase by approximately 2ºC. However, it also happens that cloud distribution and optical properties change in response to increases in temperature, and the amount of water vapor increases, modulating the increase in the average temperature at the Earth's surface.
*2 The amount of cloud water is a special mass or concentration of water particles forming a cloud. It is expressed as grams of water contained in 1m3 of air.

Frontier Newsletter/No.23
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