What causes climate change?
(Image: Cody Whitelaw: Lake Pukaki)
Water (H2O): vapour & clouds
- 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.
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
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
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.
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.
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
- National Science Challenges: Deep South Challenge
- NIWA: climate mapping
- NIWA: climate change hazards
- NASA: cloud formation
- NOAA: ocean heat content
- 2020: Bjordal et al; Equilibrium climate sensitivity above 5°C plausible due to state-dependent cloud feedback, Nature Geoscience 13, 718–721
- 2020 Bjordal et al article in Carbon Brief explaining the implications: How declining ice in clouds makes high ‘climate sensitivity’ plausible
- 2020: Zelinka et al: Causes of Higher Climate Sensitivity in CMIP6 Models, Geophysical Research Letters 47/1
- 2020 article on the above research paper: Why Clouds Are the Key to New Troubling Projections on Warming Yale 360 (Yale School of Forestry & Environmental Studies)
- 2019 Fowler et al: The effect of plant physiological responses to rising CO2 on global streamflow. Nature Climate Change, 9, 873–879
- 2019 NIWA New Zealand Fluvial and Pluvial Flood Exposure; prepared for Deep South Challenge
- 2019 Schneider et al: Possible climate transitions from breakup of stratocumulus decks under greenhouse warming Nature Geoscience 12, 163–167
- NASA explanation of the above paper: Clouds, Arctic Crocodiles and a New Climate Model
- 2018 Climate Brief explainer: What climate models tell us about future rainfall
- 2018 Climate Brief guest post from Prof. Slater: Why clouds hold the key to better climate models
- 2017: Loomis; Trees in the Amazon make their own rain, Science article
- 2013 IPCC: Clouds and aerosols In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
- 2012: Durack et al: Ocean Salinities Reveal Strong Global Water Cycle Intensification During 1950 to 2000 Science 2336: 6080, 455-458
- 2011 NIWA: Drought in a changing climate