Week 11 - Climate Prediction and Modelling

What is the aim of a climate model?

To capture and represent all the interconnected processes relevant to the Earth’s climate system (needing to simulate how the oceans, land surface and cryosphere are changing as well as the interactions between those systems)

How has increased computing power affected climate modelling?

Climate modellers have been able to incorporate more parts of the climate system into their models and to increase the overall resolution of these models

What are some important features of climate models that indicate changing patterns due to climate change?

• Volcanic eruptions temporarily cool the global climate

• Warming is more rapid over polar regions

• Warming is greater over land than oceans

• Year-to-year variability exists but there is still a clear trend due to climate change

How are climate predictions able to be made despite the significant uncertainty present due to climate change?

• We need to make different socioeconomic predictions to develop different plausible emissions scenarios

• Different factors do not matter as much to weather prediction models

• SSP = shared socioeconomic pathway = different storylines based on different changes of different factors on greenhouse gas emissions

What are shared socioeconomic pathways (SSPs)?

• Intended to span the range of plausible futures for global society, demographics and economics over the next century

• Different societal choices under each SSP are used to derive likely GHG emission scenarios and influences on human populations and their responses

What do climate scientists argue about the complexity of using SSPs as a part of climate modelling?

We should not be adding an extra layer of qualitative uncertainty

What do the SSPs currently predict about greenhouse gas emissions and their impact on the future of human populations?

• Difference of about 3°C in global temperature estimates by 2100 between a high GHG emissions pathway and a low-end pathway based on reduced emissions (despite the significant uncertainty present within a given pathway)

• Difficult to predict what actions humans may take to mitigate the effects of climate change if it were to worsen beyond our control and reach extreme levels

What are internal influences on Earth’s climate?

Govern energy transfer between different parts of the climate system (e.g. ENSO)

How is ENSO an internal influence on Earth’s climate and how is the impact of ENSO measured and assessed?

• Seasonal variations over decades and centuries

• Involves the transfer of energy between the atmosphere and ocean so it modulates the global average surface air temperature (SAT) we measure

• During El Nino events, energy is transferred from the ocean to the lower atmosphere, raising the SAT by about 0.1°C (opposite impact for La Nina events)

What are external influences on Earth’s climate?

Affect the input and output of energy to the system

What are some examples of external influences on Earth’s climate?

• How much energy the Sun is producing and Earth’s orbital characteristics

• Volcanic eruptions which produce particles that can prevent solar radiation from entering the climate system

• Human-induced greenhouse gas emissions which enhance the greenhouse effect

• Human-induced aerosol production which can reducing incoming solar radiation (from fossil fuels, geo-engineering etc)

What influences on Earth’s climate do SSPs measure?

They prescribe external influences but any spread of outcomes for a given SSP must be due to internal influences as well as variability between different models

How do different climate models differ despite being governed by the same equations?

• Different widths and heights of grid boxes (resolution - different scales of movement monitored)

• Different treatment of aerosols

• Different ways of managing processes that occur on smaller scales

• Leads to slight variations in the amount of warming in different models, given the same external influences

What is climate sensitivity?

• The uncertainty created from the difference in how much warming climate models produce, given the same GHG concentrations

• Typically defined by the amount of global warming we could expect after a doubling of atmospheric CO2

• Creates difficulty to decide what response to have from the perspective of governments)

What are our current conclusions based on climate models?

• Emissions will definitely increase into the future but the exact increase is something that we remain uncertain about

• The most recent estimate of climate sensitivity is a likely range of 2.5 to 4°C and a very likely range of 2 to 5°C (according to CMIP)

What could be a way to move forward and reduce the uncertainty of climate models?

Have an ensemble of climate model simulations with different initial conditions or different scenarios as external factors become more important in the long-term

How can climate modelling used by the IPCC become a sustainable approach to climate modelling in the future?

• Multiple models from institutions across the world

• Each model is run with the same experiments and allows for comparisons to be made between the models a few times during the running time

How is the climate changing based on current data?

• When we re-run climate models and remove the changing human influence on the climate, we don’t stimulate the same warming

• We can attribute global warming over the 20th century to human influences

How have human influenced climate change according to the IPCC?

• Warmed the climate at a rate that is unprecedented in at least the last 2000 years

• None of us experience the global average temperature so we also wanted to see how regional and local climates are changing as we experience the effects of global warming and climate change through changing local weather patterns)

What are the different signs of climate change we have observed beyond warming temperatures?

• Changing rainfall patterns (increased moisture in warmer atmosphere leads to more extreme rainfall events e.g. floods, cyclones, droughts etc)

• Rising sea levels (from thermal expansion of the ocean)

• Coral bleaching

• Changes in animals and biodiversity

• Retreating glaciers

• Changes in weather extremes and vegetation

• Melting sea ice

Why do we predict more intense short-duration extreme rainfall in the future?

• Clausius-Clapeyron relation - a warmer atmosphere can hold more moisture (especially in the tropics - already experience heavy rain) but climate change can intensify this rainfall

• Exponential relationship between air temperature and how much moisture the air can hold

• For every 1°C of warming, the air can hold about 7% more moisture

• This does not mean that all extreme rain events in the present and future can be linked to climate change

How will future sea level rise affect populations?

• Sea levels have already risen approximately 20 cm since the mid-1800s

• A further 40 to 70 cm is likely for this century and beyond

• Coastal areas such as the Pacific Islands, Maldives and coastal Australia among others where significant proportions of the population inhabit are likely to be severely impacted by global sea level rise

• Infrastructure may not be able to handle the increase in sea level rise and chance of more natural disasters from extreme weather events (e.g. flooding)

How will the climate change in the future?

• More heatwaves and fewer cold extremes - In the first 15 years of this century Australia saw 12 times as many new hot temperature records, as cold records

• More intense short-duration extreme rainfall (although droughts will still occur)

• More droughts in some places - changes in overall global circulation leading to a cluster of high pressure systems over areas that are already drought-prone

• Further melting of sea ice

• Further bleaching and death of coral due to marine warming

How will melting sea ice affect polar regions?

• Arctic sea ice is already declining and expected to keep decreasing

• The Arctic Ocean will likely become practically sea ice free during the seasonal sea ice minimum for the first time before 2050 in all considered SSPs

• No tipping point for this loss of Arctic summer sea ice

• Approximately 1 to 2 million km of sea is lost every single year

How will coral bleaching from warm sea temperatures affect marine environments such as the Great Barrier Reef?

• In 2016 and 2017 - most sites on the Great Barrier Reef experience severe coral bleaching which led to about half of the shallow coral dying (only some resurgence after this event)

• Under continued global warming, the likelihood of temperatures associated with coral bleaching on the Great Barrier Reef increases (despite the resilience of these marine ecosystems)

What was the Paris Climate Agreement of December 2015?

Aim of maintaining global average temperatures to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change

What was the purpose of the Paris Climate Agreement?

• To hold countries to account for their actions towards mitigating climate change

• Whether we achieve the aims of the Paris Agreement will determine how bad the impacts of climate change will be (we are currently not doing a great job in making progress towards this goal)

• Sustained rapid reductions are needed to meet the Paris Agreement limits (significant changes in energy use and production)

What is our current progress on mitigating the impacts of global warming?

• Today we have experienced about 1.2°C global warming due to human activities and we are on track for global warming around 3°C if we continue on this trajectory

• We have already used up almost all of our carbon budget for 1.5°C global warming and the lag times for GG means that effort to reduce CO₂ must begin now

• Global GG emissions have been rising pre-pandemic but fell sharply in 2020, sustained rapid reductions are needed to meet the Paris Agreement limits

• We have slowed down GG emissions but we must now REVERSE the trend

• The main benefits from keeping global warming to 1.5°C instead of 2°C is the reduced frequency and intensity of heat extremes which could occur through social change, radical movements and political will

How can climate scientists analyse how climate change affects extreme weather through event attribution?

• Comparing model simulations of the world including human and natural influences, where simulations where the human influences are removed (these predictions are not as accurate as they could be)

• Construct statistical distributions of our weather type of interest

• Estimate how much climate has altered the probability of an extreme event

What has event attribution in Australia revealed about likely changes in heatwaves due to climate change?

• Current conditions - 3 days above seasonal average temperature

• Projected future conditions - the spread of heatwave days will move from the extreme North to further down becoming 10 to 12 days in coastal areas such as Brisbane

What has event attribution in Australia revealed about likely changes in drought due to climate change?

• Current conditions - Perth is is set to spent a significant amount of time in drought but is an important region for growing vital crops

• Projected future conditions - major wheat growing areas in Australia are affected by the drought more than 50% of the year

What has event attribution in Australia revealed about likely changes in the Fire Danger Index due to climate change?

• Projected future conditions - most of central Australia will be affected

• Expansion to East and South-East Australia which will have damaging impacts due to the high population density and thick forest cover

What has event attribution in Australia revealed about likely changes in flood risk due to climate change?

• Current conditions - highest in the Tropics and East Coast

• Projected future conditions - South-Western and Western Australia (including Victoria) will become drier and at a greater risk of experiencing drought compared to previous times