By Megan Clark and Robert Vertessy
Australia’s land and oceans have continued to warm in response to rising CO2 emissions from the burning of fossil fuels.
This is the headline finding in the State of the Climate 2012, an updated summary of Australia’s long term climate trends released by CSIRO and the Bureau of Meteorology today (14 March 2012).
The long-term warming trend has not changed.
Each decade has been warmer than the previous decade since the 1950s. Global-average surface temperatures were the warmest on record in 2010 (slightly higher than 2005 and 1998). 2011 was the world’s 11th warmest year and the warmest year on record during a La Niña event. The world’s 13 warmest years on record have all occurred in the past 15 years.
On land around Australia the observed warming trends are consistent with the global-scale warming – despite 2010 and 2011 being the coolest years recorded in Australia since 2001.
In the oceans around Australia, sea-surface temperatures have increased faster than the global average, and sea-level rise since 1993 is greater than, or equal to, the global average.
Australian average temperatures over land
Australian annual-average daily mean temperatures showed little change from 1910 to 1950 but have progressively warmed since, increasing by 0.9 °C from 1910 to 2011. The average temperature during the past 10 years has been more than 0.5 °C warmer than the World Meteorological Organization’s standard 1961-1990 long-term average. This increase continues the trend since the 1950s of each decade being warmer than the previous.
The warming trend has occurred against a backdrop of natural, year-to-year climate variability. Most notably, El Niño and La Niña events during the past century have continued to produce the hot droughts and cooler wet periods for which Australia is well known. 2010 and 2011, for example, were the coolest years recorded since 2001 due to two consecutive La Niña events.
Rising sea level
Global-average mean sea level for 2011 was 210 mm (± 30 mm) above the level in 1880. The observed global-average mean sea-level rise since 1990 is near the high end of projections from the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report.
Rates of sea-level rise are not uniform around the globe and vary from year to year. Since 1993, the rates of sea-level rise to the north and northwest of Australia have been 7 to 11 mm per year, two to three times the global average, and rates of sea-level rise on the central east and southern coasts of the continent are mostly similar to the global average. These variations are at least in part a result of natural variability of the climate system.
Increasing sea-surface temperatures
Sea-surface temperatures in the Australian region in 2010 were the highest on record, with nine of the months during 2011 ranked in the top ten warmest months on record. Sea-surface temperatures averaged over the decades since 1900 have increased for every decade. Terrestrial and ocean surface temperatures have shown very similar warming trends over the last century.
The warm sea-surface temperatures in 2010-11 were strongly influenced by La Niña. Ocean temperatures around Australia were warmer during 2010-11 than for any previously identified La Niña event, likely due to the influence of the long-term warming trend of the past century.
Carbon dioxide (CO2) emissions account for about 60 per cent of the effect from anthropogenic greenhouse gases on the earth’s energy balance over the past 250 years. These global CO2 emissions are mostly from fossil fuels (more than 85 per cent), land use change, mainly associated with tropical deforestation (less than 10 per cent), and cement production and other industrial processes (about 4 per cent). Australia contributes about 1.3 per cent of the global CO2 emissions. Energy generation continues to climb and is dominated by fossil fuels – suggesting emissions will grow for some time yet.
CO2 levels are rising in the atmosphere and ocean.
About 50 per cent of the amount of CO2 emitted from fossil fuels, industry, and changes in land-use, stays in the atmosphere. The remainder is taken up by the ocean and land vegetation, in roughly equal parts.
The extra carbon dioxide absorbed by the oceans is estimated to have caused about a 30 per cent increase in the level of ocean acidity since pre-industrial times.
The sources of the CO2 increase in the atmosphere can be identified from studies of the isotopic composition of atmospheric CO2 and from oxygen (O2) concentration trends in the atmosphere. The observed trends in the isotopic (13°C, 14°C) composition of CO2 in the atmosphere and the decrease in the concentration of atmospheric O2 confirm that the dominant cause of the observed CO2 increase is the combustion of fossil fuels.
Australian average temperatures are projected to rise by 0.6 to 1.5 °C by 2030 when compared with the climate of 1980 to 1999. The warming is projected to be in the range of 1.0 to 5.0 °C by 2070 if global greenhouse gas emissions are within the range of projected future emission scenarios considered by the Intergovernmental Panel on Climate Change. These changes will be felt through an increase in the number of hot days and warm nights, and a decline in cool days and cold nights.
Climate models suggest long-term drying over southern areas during winter and over southern and eastern areas during spring. This will be superimposed on large natural variability, so wet years are likely to become less frequent and dry years more frequent. Droughts are expected to become more frequent in southern Australia; however, periods of heavy rainfall are still likely to occur.
Models generally indicate an increase in rainfall near the equator globally, but the direction of projected changes to average rainfall over northern Australia is unclear as there is a lack of consensus among the models.
For Australia as a whole, an increase in the number of dry days is expected, but it is also likely that rainfall will be heavier during wet periods.
It is likely (with more than 66 per cent probability) that there will be fewer tropical cyclones in the Australian region, on average, but the proportion of intense cyclones is expected to increase.
CSIRO and the Bureau of Meteorology will continue to provide observations, projections, research, and analysis so that Australia’s responses are underpinned by science of the highest quality.
A list of peer-reviewed references underpinning State of the Climate 2012 can be found on the CSIRO website.
Rob Vertessy is acting director at the Australian Bureau of Meteorology
Megan Clark is chief executive officer at CSIRO