Since the 1900s the world has warmed an average of 0.85 degrees and the sea has risen an average of 19 cm. So affirms the 2,216-page draft report
of the Intergovernmental Panel on Climate Change
(IPCC) released on 27 September 2013 (see separate FlagPost
on the IPCC). The first instalment of the IPCC’s Fifth Assessment Report (AR5) presents the latest science on global climate change. But Australia has a unique climate, influenced by both Indian Ocean events and the highly variable El Niño Southern Oscillation (ENSO) phenomenon. What does the latest IPCC report say about Australia?
Understanding IPCC scenarios
In order to model the climate system and propose possible futures, climatologists use a consistent set of scenarios. Scenarios are socio-economic ‘storylines’ that suggest greenhouse gas emission trajectories. These trajectories are used as inputs into computational models. In AR5 the possible futures are referred to as Representative Concentration Pathways (RCPs). RCPs are radiative forcing curves to 2100, where radiative forcing is a measure of atmospheric energy imbalances, measured in watts per square metre (W/m²). Four RCPs are used in AR5 (Table). The larger the number of watts, the hotter (on average) the world will be.
Greenhouse gas emissions in parts per million (ppm)
rises to 8.5 W/m² in 2100
rises to and stabilises at 6 W/m² in 2100
rises to and stabilises at 6 W/m² in 2100
peaks at 3 W/m² and declines to 2.6 W/m² by 2100
By mid-century (under RCP4.5) modelling suggests a median average temperature increase of 1.5 to 2⁰C over most of Australia, with that value slightly lower (1 to 1.5⁰C) along the coasts. This is compared to the 1986–2005 average. By 2100, modelling shows the temperature increase is up to 2⁰C on the coasts and 3⁰C elsewhere.
Under a worst-case scenario (RCP8.5) by 2100 average temperatures in Australia’s north are projected to rise by almost 5⁰C (and potentially up to 7⁰C) from a 1986-2005 baseline and by 4⁰C (to as much as 6⁰C) in the south.
Already, winter rainfall has decreased in southwest Western Australia and autumn rainfall has decreased in south-eastern Australia. The draft report predicts with at least 66% certainty that in future there will be less winter rain over southern Australia.
Sea level rise around Australia
AR5 confirms that a global average 19 cm sea level rise is due to two main factors: the thermal expansion of ocean water, and the melting of glaciers and ice sheets. According to AR5, it is 99% certain that sea level will continue to rise during the 21st century and beyond, even if greenhouse gas concentrations are stabilised. A global mean sea level rise of around 43 to 73 cm (compared to the 1986-2005 baseline) is projected by the end of this century.
Modelling (RCP4.5) suggests that the projected sea level rise around Australia will be consistent with global projections. However, observed sea-level rise in recent years has varied considerably around Australia. For example, the observed mean rate of sea level rise around north-western Australia of 8-10 mm per year since 1993 has been significantly higher than the global rate of 3.2 mm per year.
According to AR5, by 2100 in Australia there will be more warm days and nights and fewer cold ones. Under a worst-case scenario (RCP8.5):
- Both the coldest and hottest days of the year are predicted to be between 2 and 6⁰C hotter (depending on the region) than the 1981-2000 average.
- The number of nights that stay above 20⁰C is expected to increase by 60 in parts of southern Australia, and by as much as 90 to 100 in eastern Queensland. This is relative to 1981-2000, for which the national average was around 80 nights per year (from Bureau of Meteorology data).
- The number of nights that stay below 0⁰C is expected to decrease by up to 5 per year in most of southern Australia compared to 1981-2000.
Also, some extreme weather events have become more frequent and/or severe as a result of climate change, and will continue to do so. In Australia, the frequency of heat waves has increased, and AR5 predicts that there will be more droughts in southern parts of Australia.
Globally, cyclones are likely to be more intense (higher wind speeds and more rain) in the future, but their frequency is likely to either decrease or remain unchanged. Region-specific projections are not yet well quantified, so it is difficult to draw any further specific conclusions on the impacts of climate change on tropical cyclones in the Australian region.
Post co-authored by Anita Talberg and Sophie Power