What causes climate change?

What causes climate change?

Menu: The causes

(Image: Cody Whitelaw: Lake Pukaki)

Water (H2O): vapour & clouds

Summary

  • For every degree of warming, the atmosphere holds about 7% more water. This single factor is the primary driver of how climate change impacts all aspects of our lives.
  • Water vapour is the strongest greenhouse gas, accounting for 60% of warming. HOWEVER, it’s not an anthropogenic forcing. It’s driven by and in turn amplifies the effect of other greenhouse gasses. As temperature increase the atmosphere contains more water vapour. This feedback effect leads to even more warming, more evaporation, and so on.
  • Water vapour does this because heat radiated from Earth’s surface is absorbed by water vapour molecules in the lower atmosphere. The water vapour molecules, in turn, radiate heat in all directions.
  • Clouds form when water molecules condense onto a surface that’s warmer than the air: dust, soot, salt crystals etc. Clouds are the biggest uncertainty in climate models as they both shade the Earth and cool it and they also trap heat. Which effect occurs depends on the type of cloud, how reflective they are, how high they are in the atmosphere, whether it’s day or night, and the percentage of ice crystals vs. water molecules in them.

“Marine stratus and stratocumulus clouds (Fig. 1) predominantly cool the Earth. They shade roughly a fifth of the oceans, reflecting 30-60% of the solar radiation that hits them back into space. In this way, they are reckoned to cut the amount of energy reaching the Earth’s surface by between 4-7%.

“But it seems increasingly likely that they could become thinner or burn off entirely in a warmer world, leaving more clear skies through which the sun may add a degree Celsius or more to global warming…the revised estimates make the prospect of keeping global warming below 2°C—let alone the aspiration of 1.5°C agreed in the Paris climate accord five years ago—even more unlikely.”  – Fred Pierce, Yale 360

The changing makeup of clouds might mean Earth’s climate is more sensitive to increasing atmospheric carbon dioxide than currently thought.

As the climate warms, cloud ice is gradually being replaced with water – a change that has an overall cooling effect. But what happens when there is no cloud ice left? Our climate model simulations suggest that we then reach a state where warming accelerates.” – Carlsen et al.

Fig. 1: Stratocumulus clouds
Fig. 1: Stratocumulus clouds

Stratus and stratocumulus are only a few of the many types of clouds. Other types include those that form quickly and can dump larger quantities of rain, hail, and snow in short periods. Rather than acting as sun shades, these types of ‘weather bomb’ clouds may become more common.

‘Turbocharging’ the hydrology cycle

“This century, climate change will alter New Zealand’s natural water cycle significantly. It will change how much rain and snow we receive, and at what time of year. It will change how much water is stored in the soil, snow, glaciers and aquifers. It will change how much water evaporates back to the atmosphere and how much flows through streams and rivers to the coast. And it will change the severity of droughts, floods and power shortages.” – National Science Challenges

The atmosphere can hold ~7% more moisture for every degree Celsius of warming. That means more evaporation in areas that are already dry, and also increased precipitation (rain, hail, snow etc) in regions that already receive high rainfall. Higher sea surface temperatures mean there’s more water vapour over the oceans. As New Zealand is surrounded by the ocean, this increases the risk of heavy rain and snow fall and also for tropical storms to reach further south. This brings ever increasing risks for extreme flooding events. Yet with higher evaporation and transpiration, areas that are now prone to drought are likely to see longer and more intense droughts.

Impacts on New Zealand

A turbocharged hydrology means that every aspect of weather is also turbocharged; every aspect of our lives will continue to feel increasingly damaging impacts.

For this reason, multiple scientific projects are currently underway to assess the type, scale and cross-sector impacts, from river flows, loss of glaciers, farming, floods, extreme weather events, to the costs of insurance policies and how local councils need to plan for these changes. The following links will take you to key pages, each of which lists several projects in different sectors under an overarching New Zealand Government mandated project:

National Science Challenges

Explainers

Climate forcing:

The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives (negative RFS), the planet cools. Conversely, if less energy escapes than gets in (positive RFs), the planet warms.

Different climate forcings each determine how much solar energy arrives and escapes.

  • Natural Forcings are those that happen through natural changes.
  • Anthropogenic Forcings are those due to human activities.

Click here to learn about the main forcings and how they work (links to another page on this website).


Condensation and Evaporation:

Evaporation occurs more frequently at higher temperatures because the water molecules are moving more quickly.

Condensation is the opposite. Water molecules bring heat energy with them, so the surface of the dust warms slightly while the temperature of the surrounding air cools slightly, allowing the droplets to condense. This is due to a fundamental law of thermodynamics.

See the Clausius-Clapeyron Equation for describing a discontinuous phase transition between the different states (gas, liquid, solid) of water.


Transpiration:

This is the process by which plants ‘exhale’ water vapour through their stomata. Plants lose more than 90% of their water through transpiration. However, in the last 150 years as CO2 has been increasing, the density of stomata in some plants has dropped 34%. This is restricting the amount of water vapour the plants release. This has implications for the water cycle, especially in tropical rainforests, which by definition create rain largely through transpiration. This could also lead to more flooding:

Plants get more water-efficient and leak less underground soil moisture out through their pores in a carbon-rich atmosphere. Add this up over billions of leaves in very sunlit, leafy places, especially the tropics, and it means there is a bunch more soil moisture stored up underground, so much so that climate models predict rainfall events will saturate the ground and more rain will run off into rivers.”  – Ass. Professor Mike Pritchard, UCI

References and further reading