Understanding climate change

Carbon dioxide (CO₂) is the most prevalent greenhouse gas in the atmosphere (after water vapour), contributing the most to radiative forcing. Human emissions of CO₂ from fossil-fuel combustion, industrial processes (such as cement production) and deforestation are adding CO₂ to the atmosphere faster than it can be taken up by the land biosphere and oceans, increasing atmospheric concentrations.

Methane (CH₄) is the next most important greenhouse gas in terms of impact on the radiative imbalance, contributing about 17%. Although at a lower concentration in the atmosphere, CH₄ has a much higher warming effect than CO₂ for a given mass.

Approximately 40% of CH₄ is emitted into the atmosphere from natural sources, such as wetlands, and 60% from human activities, such as ruminant livestock production, rice agriculture, fossil-fuel extraction, landfill waste and biomass burning. Atmospheric concentrations of CH₄ have increased by more than 170% since industrialisation to 2,017.6 parts per billion (ppb) in August 2017.

Recent increases are thought to be from emissions from wetlands in the tropics in response to global temperature increases and from human activities at mid-latitudes of the northern hemisphere. Methane is removed from the atmosphere through chemical degradation.

Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which are used in refrigeration and air-conditioning systems, contribute about 11% to radiative forcing.

Although CFC concentrations are decreasing in the atmosphere as a result of international action to protect the ozone layer, concentrations of HCFCs and HFCs, although less damaging to the ozone layer, are being used to replace CFCs, and are rapidly increasing. These are potent greenhouse gases with much longer atmospheric lifetimes than CO_2.

Nitrous oxide (N₂O) has contributed about 6% to the radiative forcing caused by long-lived greenhouse gases, with concentrations having increased by 22% from 270 ppb since industrialisation to 328.9 ppb in August 2017. Agriculture is the main source of N₂O emissions associated with human activity, including from soil cultivation, fertiliser use and livestock manure management. Fossil-fuel burning also produces N₂O.

Our current climate is changing far more rapidly now than in the past. Global mean air surface temperature¹ has increased by over 1°C since the industrial revolution (Figure 9). It has been shown that 2016 was the second year in a row global temperatures were over 1°C above the 1850–1900 average and 2017 was the third warmest year on record. Each of the most recent three decades has been warmer than all preceding decades since 1850. The last decade has been the warmest of these.

The 10 warmest (global combined land and ocean annually averaged temperature) years on record since 1880 to 2017 all occurred since 1998, with the 5 warmest all since 2010 (Table 2).

¹ Combined land and ocean surface temperature.

Table 2: Global combined land and ocean annually averaged temperature rank and anomaly

Source: US National Centres for Environmental Information ‘State of the climate’ section

On 12 December 2015, nations agreed (The Paris Agreement) to pursue efforts to limit average global temperature rise this century below 2°C above pre-industrial levels and to strengthen efforts in pursuit of a 1.5°C target. This is considered the limit beyond which the effects of climate change will have dangerous risks and impacts for humans and ecosystems, with potentially irreversible changes in the climate system.

A 2°C warming may not sound significant – we are used to greater temperature fluctuations on a day-to-day basis. However, normal weather variability should not be confused with a sustained increase in average global temperature.

The difference in average global temperature between an ice age and an interglacial period is only 5°C. The characterisation of 2°C as the threshold between acceptable and ‘dangerous’ climate change is premised on an early assessment of the scope and scale of the accompanying impacts. More recent research has revised the impacts associated with an average 2°C global warming and 2°C is regarded as an inadequate limit.  

Changes consistent with an increase in global temperature have been observed in many other parts of the climate system. More than 90% of the total heat accumulated in the climate system between 1970 and 2010 has been stored in the ocean, causing ocean temperatures to rise. Mountain glaciers have been shrinking and the Greenland and West Antarctic ice sheets have lost ice, contributing to sea-level rise.

Atmospheric and ocean circulation is changing, the acidity of ocean water is increasing and an increasing number of plants and animals are undergoing shifts in distribution and life cycles consistent with observed temperature changes.

Arctic sea ice is shrinking and thinning, with the summer ice volume in October 2017 a staggering 65% below the ice volume in October 1979. An ice-free Arctic in the warmer months is possible as early as 2020, with the ice-free period lengthening over time.

Loss of the Arctic ice represents more than an ecological upheaval in the north of the world. It will have profound global climatic effects that are already intensifying climate change and have the potential to destabilise the climate system.

Figure 10 shows the mean annual cycle of total Arctic sea ice volume from 2010–18 (shaded areas indicate 1 and 2 standard deviations from the mean).

The magnitude of future climate change depends on the concentration of future greenhouse gas emissions and how the climate system responds to the additional warming.

As the main greenhouse gases have long lifetimes in the atmosphere, reducing emissions will only slow the rate of increase of atmospheric concentrations, rather than stabilise them. 

Stabilising atmospheric concentrations requires emissions to be reduced to very near zero and even, depending on timescales and pathways to stabilisation, for some existing greenhouse gases to be actively removed from the atmosphere. The science-based carbon budget approach highlights that long-term, gradual reductions in emissions are insufficient. Rapid and deep reductions are required if the global temperature increase is to be limited to below 2°C.

The amount of global emissions consistent with the 2°C target is being rapidly consumed, with only 2 to 3 decades remaining before the global economy must achieve net zero emissions.

Many aspects of climate change will continue for centuries, even if emissions of greenhouse gases due to human activities, are stopped. As parts of the climate system respond slowly, such as the deep ocean and ice sheets, change will continue long after emissions cease. For example, oceans will continue to remove heat from the atmosphere in deeper ocean layers and this process will further warm the oceans for centuries.

The Intergovernmental Panel on Climate Change² (IPCC) has modelled projections of future levels of greenhouse gas emissions based on different models of population size, economic activity, lifestyle, energy use, landuse patterns, technology and climate mitigation policy.

The representative concentration pathways (RCPs) describe 4 different scenarios of greenhouse gas emissions and the resulting radiative forcing by the end of the 21^st century. These are: RCP8.5, RCP6, RCP4.5 and RCP2.6, with the number indicating the forcing measured in watts per square metre (m²). The estimate of the current level of forcing from long-lived greenhouse gases is around 3 W/m².

² The IPCC was jointly established by the World Meteorological Organization and the United Nations Environment Programme to assess the scientific, technical and socio-economic information relevant for understanding the risk of human-induced climate change.

Scenario RCP2.6

This is the most severe mitigation scenario with emissions peaking around the year 2020 then rapidly declining. The RCP2.6 scenario is the only one that aims to keep global warming below 2°C above pre-industrial temperatures. In this scenario, we would see warming significantly reduced later this century and beyond. The CO₂ concentration reaches 440 ppm by 2040 then slowly declines to 420 ppm by 2100. This pathway requires early participation from all emitters and for technologies to be applied for actively removing CO₂ from the atmosphere.

Scenarios RCP4.5 and RCP6.0

These are the 2 intermediate emissions scenarios. The RCP4.5 scenario reflects emissions peaking around 2040, with the CO₂ concentrations reaching 540 ppm by 2100. The RCP6.0 scenario represents some mitigation effort with CO₂ concentrations continuing to rise to 660 ppm by the end of the century.

Scenario RCP8.5

This represents a future with some or very little curbing of emissions, with CO₂ concentrations continuing to rapidly rise to 940 ppm by 2100. The high emissions pathway RCP8.5 is expected to result in a global average warming of around 4.5°C by 2100 with an uncertainty range of 3°C to 6°C. The impacts on society associated with a 4°C warming present great challenges for adaptation.

What does this mean

Present emission levels are tracking close to the highest scenario – RCP8.5

A shift to a 2°C pathway requires immediate significant and sustained global mitigation, based on collective local, national, and global action, probably relying on CO₂ removal and negative emission approaches.

There are significant risks associated with removal technologies, including uncertainty in their carbon retention, consequences of large-scale deployment, costs and feasibility.

Given the difficulty of achieving rapid and large reductions in global emissions, we need to increase our understanding of the impacts of high-end climate change and the implications these have for adaptation planning.

A joint report by the CSIRO and BOM projects that by 2030, Australian annual average temperature will increase by 0.6–1.3 °C above the climate of 1986–2005. The projected temperature range by 2090 is projected to be between 0.6–1.7 °C and 2.8–5.1 °C, depending on the scenario.