Global climate models are mathematical representations of the Earth’s climate system, based on the laws of physics and run on powerful computers. They represent fundamental physical processes in the atmosphere, ocean, land surface and cryosphere (frozen water part of the Earth system ). 

Theoretically, these physical processes can be represented mathematically even though there is some chaotic behaviour involved at very small scales (e.g. water droplet interactions within clouds). 

Some of the challenges in developing an Earth system model are that:

(a) many processes making up the Earth's climate operate on different time and space scales, from a few seconds to thousands of years, from a few metres to thousands of kilometres; and

(b) these processes interact with each other across different time and space scales. This means that even though a global climate model is mainly used for decade-to-century long projections, they try to include short-term and small scale processes since these also interact with the larger scales.

At each model time step, a new state of the Earth’s atmosphere and oceans is calculated and then used as the initial state for the next time step. By this method the model is ‘stepped forward’ in time. The simulated climate can then be inferred by the ‘statistics’ (averages, extremes etc) from multi-decade simulations. Typical global climate model time steps are 30 minutes up to 3 hours – meaning that processes that happen on shorter time scales are not captured, but must be approximated. For spatial scales: with typical spatial resolution of 200km (for the atmosphere) for each grid cell of the model, processes that are smaller in scale (e.g. condensation of water vapour into cloud droplets), are not simulated but are approximated. 

A significant constraint is the cost involved in running such complex models. Usually run on powerful super computers, the costs are not only the runtime, but also the infrastructure necessary for storing such large data amounts.

Despite these limitations, global climate models manage to produce quite realistic daily weather, seasonal variability and long term climate states.

Some of the important parts of a global climate model relate to:

1. The response to changes in solar radiation on time scales.

2. Changes to the Earth's energy balance at the surface and top of atmosphere from volcanic eruptions

3. How radiation is absorbed and reflected on its way through the atmosphere and at the surface.

4. Atmosphere and ocean dynamics (and how energy is transported through them)

5. How greenhouse gases and aerosols affect the Earth's climate 

6. Sea ice and polar ice sheets

7. Various climate ‘feedback’, such as the interaction of clouds and water vapour with the warming climate, and the changing absorption or emission of CO2 from the ocean and land surface.

The future of human caused greenhouse gas and aerosol emissions is highly uncertain, due to substantial unknowns in population and economic growth, technological developments, and political and social changes.

The climate modelling community has developed Representative Concentration Pathways (RCPs) to explore credible future options. The Australian climate change projections found on this site are derived from climate models from the RCPs.

These scenarios span the range of plausible global warming scenarios. They provide a range of options for the world’s governments and other institutions for decision making.

RCPs are pathways for greenhouse gas and aerosol concentrations, together with land use change, that are consistent with a set of broad climate outcomes used by the climate modelling community. The pathways are characterised by the radiative forcing produced by the end of the 21st century. Radiative forcing is the extra heat the lower atmosphere will retain as a result of additional greenhouse gases.

The RCPs now explicitly include the effect of mitigation strategies (strategies to attempt to reduce the problems). No particular scenario is deemed more likely than the others, however, some require major and rapid change to emissions to be achieved.

RCP8.5 - a future with little curbing of emissions, with a CO2 concentration continuing to rapidly rise, reaching 940 ppm by 2100.

RCP6.0 – lower emissions, achieved by application of some strategies and technologies. CO2 concentration rising less rapidly, but still reaching 660 ppm by 2100 and total stabilising shortly after 2100.

RCP4.5 - CO2 concentrations are slightly above those of RCP6.0 until after mid-century, but emissions peak earlier (around 2040), and the CO2 concentration reaches 540 ppm by 2100.

RCP2.6 - the most ambitious scenario, with emissions peaking early in the century (around 2020), then rapidly declining. Such a pathway would require early participation from all emitters, including developing countries, as well as the application of technologies for actively removing carbon dioxide from the atmosphere. The CO₂ concentration reaches 440 ppm by 2040 then slowly declines to 420 ppm by 2100.

Read at https://www.climatechangeinaustralia.gov.au/en/climate-projections/future-climate/global-climate-change/causes-past-and-recent-change/  

In climate science, ‘attribution’ describes accounting for the causes of observed changes in the climate system.

Researchers typically use a combination of climate modelling, instrumental observations, studies of feedback processes and sometimes palaeoclimate reconstructions to investigate cause and effect .

Climate models can characterise both natural climate variability and changes to the climate system that are driven by factors such as increases in greenhouse gases, variations in solar radiation and emissions of volcanic aerosols. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions. 

Evidence of human influence on the climate system has strengthened over the past decades. Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes.

The Fifth IPCC Assessment Report concluded that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

Global mean temperature has risen by around 0.85 °C from 1880 to 2012, at a rate of around 0.12 °C per decade since 1951. Increasing greenhouse gases were likely to have been responsible for between 0.5 °C and 1.3 °C of warming from 1951-2010.

The IPCC also concluded:

1. It is very likely that human influence, particularly greenhouse gases and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961.

2. It is very likely that human actions have made a substantial contribution to increases in global upper ocean (0–700 m) heat content observed since the 1970s.

3. It is likely that human influences have affected the global water cycle since 1960. Human influences have contributed to observed increases in atmospheric moisture content in the atmosphere (medium confidence), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence), and to changes in surface and sub-surface ocean salinity (very likely).

4. It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heat waves in some locations.

5. Human influences have very likely contributed to Arctic sea ice loss since 1979.

6. Human influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface mass loss of the Greenland ice sheet since 1993.

7. It is likely that there has been human contribution to observed reductions in Northern Hemisphere spring snow cover since 1970.

8. It is very likely that there is a substantial human contribution to the global mean sea level rise since the 1970s.

Evidence shows that many of the effects of changing climate are already occurring. These include: 

(i) an increase in the surface water temperature of lakes and streams across Europe, especially those at high altitudes and latitudes; 

(ii) an increase in temperature of large deep lakes; 

(iii) a reduction in lake ice-cover; and 

(iv) the melting of mountain glaciers and permafrost causing changes to mountain streams and release of solutes and pollutants to surface waters. 

Climate models show that in the future it is likely that these trends will continue and it is probable that: 

(i) there will be changes in the flow regime of streams and rivers associated with projected changes in the amount, seasonality, intensity and distribution of precipitation, causing an increase in the transport of sediments and nutrients downstream to lakes and the coastal zone; 

(ii) there will be changes in precipitation, evaporation and flooding dynamics that will cause changes in water levels, habitat structure and water residence times in wetlands; 

(iii) small intermittent streams and small lakes in warm dry areas may disappear, while flow in permanent streams may become intermittent and lakes may become more salty; and (iv) systems already at a threshold between two different conditions may change abruptly, e.g. may switch from permanent to intermittent streams, from freshwater to permanently salty lakes.

Ecological consequences of climate change for freshwater ecosystems in Europe

The ecological response of freshwater ecosystems to climate change needs to allow for interactions between climate change and the many stressors already affecting rivers, lakes and wetlands. These include water resource management, eutrophication, acidification, toxic substances, catchment land-use change and invasion of exotic species. The observed and expected impacts, however, differ strongly between ecosystem types (lakes, rivers, wetlands) and climate regions.

In cold regions expected changes include:

(i) farming increase in response to an increase in the length of the growing season and more available land due to less ice-cover, and an increase in nutrient release from catchment soils; 

(ii) loss of species in response to increasing water temperature; 

(iii) changes to food-web structure which in lakes leads to higher phytoplankton biomass and a decrease in oxygen concentrations; 

(iv) adverse impacts on submerged aquatic plants caused by changes in underwater light resulting from an increase in water turbidity caused by more intense precipitation and suspended sediment loads in summer; and 

(v) increased invertebrates in rivers as a result of  increased meltwater discharge. 

In temperate and warm-humid regions freshwaters are especially vulnerable to problems of eutrophication. Climate change is expected to lead to higher farming with more intense algal blooms, stronger and longer periods of summer  with greater oxygen depletion. These environmental changes will further result in significant modifications in the distributions of species across ecoregions, higher susceptibility to alien species invasion; and overall biodiversity reduction.

In warm-arid ecoregions changes in moisture balance are expected to have severe consequences for freshwaters. Reduced precipitation coupled with increased temperature will lead to the loss of habitat and to changes in community resulting from a decrease in lake-levels, a reduction in river flow and increased eutrophication. In the dry season rivers and lakes that are currently permanent may experience intermittent drying out and those that are currently intermittent in character may disappear completely. Changes in community composition and food-web structure caused by increased salt content are also expected. Biota will be threatened by habitat loss.

Read the following information:

• https://www.theguardian.com/environment/2017/jul/10/earths-sixth-mass-extinction-event-already-underway-scientists-warn (contains video 1.72mins)

A “biological annihilation” of wildlife in recent decades means a sixth mass extinction in Earth’s history is under way and is more severe than previously feared, according to research.

Scientists analysed both common and rare species and found billions of regional or local populations have been lost. They blame human overpopulation and over-consumption for the crisis and warn that it threatens the survival of human civilisation, with just a short window of time in which to act.

The study, published in the peer-reviewed journal Proceedings of the National Academy of Sciences, does not use the normally sober tone of scientific papers and calls the massive loss of wildlife a “biological annihilation” that represents a “frightening assault on the foundations of human civilisation”.

Prof Gerardo Ceballos, at the Universidad Nacional Autónoma de México, who led the work, said: “The situation has become so bad it would not be ethical not to use strong language.”

Previous studies have shown species are becoming extinct at a significantly faster rate than for millions of years before, but even so extinctions remain relatively rare giving the impression of a gradual loss of biodiversity. The new work instead takes a broader view, assessing many common species which are losing populations all over the world as their ranges shrink, but remain present elsewhere.

The scientists found that a third of the thousands of species losing populations are not currently considered endangered and that up to 50% of all individual animals have been lost in recent decades. Detailed data is available for land mammals, and almost half of these have lost 80% of their range in the last century. The scientists found billions of populations of mammals, birds, reptiles and amphibians have been lost all over the planet, leading them to say a sixth mass extinction has already progressed further than was thought. Billions of animals have been lost as their habitats have become smaller with each passing year.

The scientists conclude: “The resulting biological annihilation obviously will have serious ecological, economic and social consequences. Humanity will eventually pay a very high price for the decimation of the only assemblage of life that we know of in the universe.”

They say, while action to halt the decline remains possible, the prospects do not look good: “All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life.”

Wildlife is dying out due to habitat destruction, over-hunting, toxic pollution, invasion by alien species and climate change. But the ultimate cause of all of these factors is “human overpopulation and continued population growth, and over-consumption, especially by the rich”, say the scientists.

“The serious warning in our paper needs to be heeded because civilisation depends utterly on the plants, animals, and microorganisms of Earth that supply it with essential ecosystem services ranging from crop pollination and protection to supplying food from the sea and maintaining a livable climate,” Ehrlich told the Guardian. Other ecosystem services include clean air and water.

“The time to act is very short,” he said. “It will, sadly, take a long time to humanely begin the population shrinkage required if civilisation is to long survive, but much could be done on the consumption front and with ‘band aids’ – wildlife reserves, diversity protection laws – in the meantime.” Ceballos said an international institution was needed to fund global wildlife conservation.

The research analysed data on 27,500 species of land vertebrates from the IUCN and found the ranges of a third have shrunk in recent decades. Many of these are common species and Ceballos gave an example from close to home: “We used to have swallows nesting every year in my home near Mexico city – but for the last 10 years there are none.”

The researchers also point to the “emblematic” case of the lion: “The lion was historically distributed over most of Africa, southern Europe, and the Middle East, all the way to northwestern India. [Now] the vast majority of lion populations are gone.”

Historically lions lived across Africa, southern Europe, the Middle East, all the way up to Northwestern India. Today their habitat has been reduced to a few tiny pockets of the original area.

Prof Stuart Pimm, at Duke University in the US and not involved in the new work, said the overall conclusion is correct, but he disagrees that a sixth mass extinction is already underway: “It is something that hasn’t happened yet – we are on the edge of it.”

Pimm also said there were important caveats that result from the broad-brush approach used. “Should we be concerned about the loss of species across large areas – absolutely – but this is a fairly crude way of showing that,” he said. “There are parts of the world where there are massive losses, but equally there are parts of the world where there is remarkable progress. It is pretty harsh on countries like South Africa which is doing a good job of protecting lions.”

Robin Freeman, at the Zoological Society of London, UK, said: “While looking at things on aggregate is interesting, the real interesting nitty gritty comes in the details. What are the drivers that cause the declines in particular areas?”

Freeman was part of the team that produced a 2014 analysis of 3000 species that indicated that 50% of individual animals have been lost since 1970, which tallies with the new work but was based on different IUCN data. He agreed strong language is needed: “We need people to be aware of the catastrophic declines we are seeing. I do think there is a place for that within the [new] paper, although it’s a fine line to draw.”

Citing human overpopulation as the root cause of environmental problems has long been controversial, and Ehrlich’s 1968 statement that hundreds of millions of people would die of starvation in the 1970s did not come to pass, partly due to new high-yielding crops that Ehrlich himself had noted as possible.

Ehrlich has acknowledged “flaws” in The Population Bomb but said it had been successful in its central aim – alerting people to global environmental issues and the the role of human population in them. His message remains blunt today: “Show me a scientist who claims there is no population problem and I’ll show you an idiot.”