Global Warming, Clouds, and Aerosols
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How Climate Change Works

Climate change refers to changes in the average state of temperature and weather patterns, such as precipitation and clouds, over decades. One cause of climate change is human activity, the focus of global warming. Another includes natural factors, such as changes in solar activity and large-scale volcanic eruptions, with their warming and cooling effects (ice ages). However, the rapid temperature rise observed since 1900 cannot be explained by natural factors alone; this is now called global warming.

Global Warming Prediction

How does climate change occur? Climate change is first caused by external factors, such as volcanic activity and anthropogenic emissions of greenhouse gases and aerosols, followed by feedback from clouds, water vapor, sea ice, land ice, and snow, and then by a transition to a new climate state (Figure 1).

Figure 1: Elements Contributing to Climate Change
Figure 1: Elements Contributing to Climate Change
(Source: IPCC-AR6/WG1, Figure. Figure 7.1)
A new, equilibrium climate state is triggered by external factors, such as volcanic activity, greenhouse gases, aerosol emissions, and feedback from clouds, sea ice, land ice, snow, and other factors. As discussed in the next section, radiative forcing is used to quantify this external factor. The quantified temperature increase is called climate sensitivity. Feedback may be positive, which accelerates warming, or negative, which work toward cooling. However, there is a large uncertainty in the quantification of cloud feedback, leading to a large uncertainty in estimates of climate sensitivity (i.e., the temperature increase).

In addition to natural causes, there are anthropogenic causes of climate change, such as carbon dioxide emissions from burning fossil fuels (e.g., coal, oil, and natural gas). Other causes are energy production and the emission of greenhouse gases, such as methane from livestock, natural gas drilling, and burning fossil fuels. Radiative forcing is a measure that quantifies the contribution of these factors to climate change, as discussed in the next section.

The mechanism that accelerates or decelerates the change is called feedback. Positive feedback accelerates warming, while negative feedback slows it. For example, warming melts sea ice, land ice, and snow. Exposed land and sea absorb more sunlight than snow and ice surfaces, raising global temperatures and accelerating warming. This is snow/ice albedo feedback. Also, as the Earth's temperature rises, the amount of water vapor in the atmosphere increases, and since water vapor is a greenhouse gas, it accelerates global warming. This is called water vapor feedback. As various feedback progress, temperatures and weather patterns settle into a different state, and the climate enters a new one.

Another common feedback is cloud feedback. Clouds, too, change as global temperatures rise and alter the climate. The next section discusses how clouds are thought to change with warming.

How do clouds change with warming?

One of the challenges in understanding climate change is predicting how clouds will change as temperatures rise due to increased carbon dioxide and other factors and whether these changes will be fast or slow. Clouds cover two-thirds of the Earth, so their impact is very large.

Since the Industrial Revolution, global temperatures have increased and cloud characteristics, such as altitude, volume, and phase (water/ice), have changed, affecting the global radiation budget and thus changing global temperatures (Figure 2). This process is called cloud feedback and can either amplify or offset the effects of temperature increase. Specifically, high-altitude clouds rise as the temperature increases, and the effect of keeping heat away from the Earth becomes more pronounced. Over subtropical oceans, low clouds are less abundant and allow more solar radiation to reach the surface. Both of these effects accelerate warming. On the other hand, rising temperatures in high-altitude clouds melt ice particles to form water droplets. In the phase change from solid ice to liquid water, the large ice grains are transformed into a large number of smaller water droplets, which have a stronger effect of blocking solar radiation from the ground, cooling the ground and slowing global warming.

In other words, part of the temperature increase that is accelerated by changes in cloud height and quantity due to temperature increases is cooled by changes in cloud phase, but clouds are believed to accelerate warming overall.

Figure 2: Changes in cloud cover and their impact on climate due to global warming
Figure 2: Changes in cloud cover and their impact on climate due to global warming
(Source: IPCC-AR6/WG1, Figure. Chapter 7 FAQ7.2 Figure 1) Global warming is believed to accelerate warming by affecting (left) cloud altitude and (center) cloud volume. Part of this is mitigated by the cooling effect associated with the cloud phase change (from ice grains to water droplets), but overall, clouds are thought to accelerate warming. A scientific understanding of how clouds change in altitude and characteristics is important for predicting warming.

Thus, to quantitatively assess the impact of clouds on warming, it is important to have a scientific understanding of which altitudes clouds move to and which cloud properties change during warming. In recent years, advances in observations and numerical modeling have led to more sophisticated studies of cloud processes, improving our understanding of how clouds behave during warming. However, the impact of clouds on warming has not been fully quantified and remains the largest source of uncertainty in global warming predictions. Thus, quantitative assessment of cloud effects on climate requires vertical observations that provide detailed cloud characteristics at each cloud altitude.

Error Factors in Climate Change Predictions - Unknown Functions of Clouds and Aerosols

Temperature conditions in the Earth's climate system, such as warming and cooling, strongly depend on the balance and transfer processes of radiant energy received from the sun and thermal energy emitted from the Earth.

Various factors influence this, including greenhouse gases, large volcanic eruptions, direct and indirect effects of clouds and aerosols on radiation, and changes in solar activity. A quantitative measure of the impact of these external factors on climate change is radiative forcing.

Radiative forcing is the effect of a factor on the Earth's radiant energy balance, expressed in units of W/m2.

Figure 3 shows the magnitude of radiative forcing for anthropogenic and natural sources of climate change. A positive value of radiative forcing contributes to warming, while a negative value contributes to cooling. The black lines on the radiative forcing graph are error bars (i.e., the likely magnitude of the error).

Figure 3 Elements contributing to radiative forcing
Figure 3 Elements contributing to radiative forcing
(Source: IPCC-AR6/WG1, Figure. Chapter 7 Figure 7.6) Positive values such as carbon dioxide (CO2 bars) contribute to warming, while negative values such as aerosols (aerosol bars) contribute to cooling. The uncertainty in the warming effect of such greenhouse gases is relatively low. On the other hand, the cooling effect of aerosols is non-negligible and contains large uncertainties, especially with respect to their interactions with clouds (light blue aerosol-cloud), which is poorly understood by the scientific community.

In this figure, the radiative forcing of greenhouse gases such as CO2 and CH4 is positive, indicating a high contribution to warming. These error bars are small, indicating that the prediction errors are small.

On the other hand, aerosol effects show negative radiative forcing and are expected to contribute to cooling. In particular, the error bars for the cloud-aerosol interaction effect are very large, indicating that the radiative forcing is poorly understood. In other words, aerosols offset some of the temperature increase due to greenhouse gases*1, but there is a high degree of uncertainty as to how much they do so, and this creates the greatest uncertainty in the overall future temperature projections. This uncertainty is likely due to a lack of scientific understanding of the effects of aerosols on the radiation budget and their interaction with clouds. Therefore, there is a need for effective observations of clouds and aerosols in order to understand their behavior on Earth and their impact on climate.

*1 Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report Summary for Policymakers:

https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf