SoE 2009 > Climate Change > 2.1 Climate change and the greenhouse effect

2.1 Climate change and the greenhouse effect

Over the last century Australia, along with the rest of the world, has experienced an average warming of about 0.9°C. It is now more than 90% certain that observed increases in global temperature are caused by greenhouse gas emissions. The projected effects of this warming include changing rainfall patterns, rising sea levels and increased evaporation. These effects are already being observed.

The average annual temperature in New South Wales is increasing at an accelerating rate. While global temperatures and atmospheric greenhouse gas concentrations have fluctuated naturally over the millennia, the rapid increase in temperatures and greenhouse gas concentrations cannot be explained by natural variation.

Global sea level rise has accelerated over the past century. A range of sea level rise projections has been modelled by the Intergovernmental Panel on Climate Change, which acknowledges that there will also be regional variability. For planning purposes, the NSW Government has adopted sea level rise benchmarks of 0.4 metres by 2050 and 0.9 metres by 2100.

The NSW Government has consulted the community while developing a new response to climate change and sea level rise in the context of the Australian Government's commitment to cap and reduce the nation's greenhouse gas emissions.

NSW indicators

Notes: Terms and symbols used above are defined in About SoE 2009 at the front of the report.

Introduction

The Earth's atmosphere has been subject to a natural greenhouse effect for some four billion years. This warms the Earth making it habitable for life. Greenhouse gases in the atmosphere, including carbon dioxide, methane, water vapour, nitrous oxide and ozone, allow solar radiation to pass through the atmosphere relatively unimpeded.

Solar energy reaches the Earth's surface and some is absorbed by the oceans, soils and vegetation. Some of this energy is re-radiated as infrared radiation (heat) and is partly trapped by the greenhouse gases in the atmosphere. This allows the atmosphere to maintain an average global surface temperature of about 14°C, which is about 33°C warmer than if there were no greenhouse gases in the atmosphere (IPCC 2007a, p.946).

Increased use of fossil fuels since 1750, land-use changes, agriculture and other activities have resulted in an increased accumulation of greenhouse gases (including chlorofluorocarbons (CFCs)) in the atmosphere. This has led to more infrared radiation being trapped as heat, resulting in an increase in global surface temperature (IPCC 2007a, p.4). Australia, as well as NSW, has now recorded a warmer-than-average year for the past 12 years (BoM 2008a).

Global temperatures and atmospheric greenhouse gas concentrations have always fluctuated naturally over the millennia. Climate change is not a new phenomenon. However, the current magnitude and rates of increase of atmospheric concentrations of greenhouse gases and temperature are unprecedented in the past 800,000 years (Lüthi et al. 2008).

In 2007 the Intergovernmental Panel on Climate Change (IPCC) published its Fourth Assessment Report, based on all current published scientific material (IPCC 2007a). Key findings include the following:

• Warming of the climate is unequivocal, demonstrated by increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea levels.
• It is 'very likely' (defined as >90% probability of occurrence) that most of the observed increases in global average temperatures since the mid-20th century are due to the observed increased greenhouse gas concentrations resulting from human-induced (anthropogenic) emissions.
• Even if greenhouse gas concentrations were to be stabilised, anthropogenic warming and sea level rise will continue for centuries, due to the time lags associated with climate processes.

Future trends in global greenhouse gas emissions will influence the type and magnitude of climate change impacts. To address uncertainty in the growth of future emissions, the IPCC has developed a number of potential scenarios (Nakicenovic et al. 2000). Each of these scenarios would result in a different concentration of greenhouse gases in the atmosphere, which would in turn lead to a variety of changes in the climate system. Recent analyses indicate that the rate of emissions growth since 2000 has been greater than any of the IPCC's scenarios (Raupach et al. 2007) and that the rate of increase in global mean surface temperatures and sea level rise is in the upper range of the IPCC's climate projections (Rahmstorf et al. 2007).

The complexity of the climate system may result in rapid and abrupt changes. Climate 'tipping points', when gradual changes to the climate system produce feedbacks, can result in abrupt climate shifts. One example of this type of feedback is the loss of ice and snow cover from the margins of Greenland. Once the reflective ice and snow are removed, the ocean and land absorb radiation at a faster rate, which leads to further melting of snow and ice. A sudden collapse of the Greenland ice sheet may, in turn, abruptly increase sea levels, which otherwise can be predicted to increase at a steady rate.

To better understand what the impacts of climate change signify for the state, the NSW Government, in partnership with the Climate Change Research Centre at the University of NSW, has developed regional climate projections for NSW (DECCW in prep.). This work has used the same data sources as were used in 2007 by CSIRO and the Australian Bureau of Meteorology (CSIRO & BoM 2007), but the data was processed using innovative methods published in the international literature. These projections have been used to assess the likely impacts of the future climate changes NSW may face by 2050.

Status and trends

Climate change background

The difference between a planetary ice age and a warm interglacial period is a variation in global average temperature of 6–7°C. Temperature changes of this scale can lead to major changes in the world's climate and ecosystems (IPCC 2007b). Approximately three million years ago, when average global temperatures were 2–3°C higher than at present, sea levels were 13–37 m higher (Pearman 2008). Climate change is therefore not a small change in the scale of natural variability – it represents a change on a scale that has triggered mass extinctions in the past. Importantly, these temperature changes are similar to the range of projections currently provided by the IPCC and the recent local study by the NSW Government and University of NSW (IPCC 2007a; DECCW in prep.).

Natural variations in temperature and rainfall in NSW are influenced by a number of climate systems in eastern Australia, including the El Niño – Southern Oscillation (ENSO), Southern Annular Mode and Indian Ocean Dipole (DECCW in prep.). Although there is natural variability in the climate, there is consensus among climate scientists that the rate and magnitude of climate change that NSW is currently experiencing are outside the expected range of this natural variability.

Changing climate – historical and projected

Temperature

The rate of warming over the last 50 years is nearly twice that for the whole 20th century (IPCC 2007b). Global warming does not mean that each year will be warmer than the last. Natural variability can cause cooling for a decade and will continue to do so throughout the 21st century (Easterling & Wehner 2009). Increased greenhouse gases cause a long-term warming trend as has been observed over Australia. Annual mean temperatures have increased by about 0.9°C since 1910, with significant regional variations (CSIRO & BoM 2007).

The average annual temperature in NSW has been increasing at an accelerating rate since the mid-1990s, based on current climate trends (Figure 2.1). The annual average temperature rise was around 0.1°C per decade during 1950–80 and since 1990 it has been about 0.5°C per decade, a five-fold increase (DECCW in prep.). Since record-keeping began in 1910, the warmest year for NSW was 2007, at 1.1°C above the 1961–90 NSW average temperature (Figure 2.1). All years from 1997 to 2008 were warmer than average, with 2008 marking the 12th consecutive year with above-average temperatures, an unprecedented sequence in the historical records (BoM 2008b). These changes are outside the natural climate variability and are 'very likely' (>90% probability) the result of increased greenhouse gas emissions from human activities (CSIRO & BoM 2007).

Figure 2.1: NSW annual mean surface temperature anomaly, 1910–2008

Source: BoM 2008b

Notes: Zero on this figure is the 1961–90 NSW temperature average. The red line represents the 11-year moving average of the measurements.

The latest IPCC report states that later in this century the climate is 'virtually certain' (>99% probability) to be warmer than at present (IPCC 2007a). CSIRO projections indicate that by 2030 average temperatures in Australia will rise by about 1°C from the current average, with average summer temperatures likely to be at least 3°C warmer by 2070 (CSIRO & BoM 2007; Pitman & Perkins 2008).

It is expected that NSW will become hotter, with an increase in maximum and minimum temperatures 'very likely' in all seasons. The north and west of the state are generally expected to see the greatest increases in maximum temperatures (Map 2.1). By 2050, winter and spring annual maximum temperatures are expected to rise by around 2–3°C across much of northern NSW (DECCW in prep.). Recent analyses have also begun to focus on temperatures which occur less frequently, such as those that occur once every 20 years. One analysis, based on climate models reproducing current observations over NSW, suggests that by 2100 daily temperatures may exceed 50°C in parts of the state every few years (Perkins et al. 2009).

Map 2.1: Projected increases in seasonal average maximum temperatures by 2050

Rainfall

In 2008 NSW experienced its eighth consecutive year of below-average rainfall. Despite this, parts of the state, including the north coast and central tablelands, had above-average rainfall (BoM 2008a). In central NSW, 2008 was a dry year and rainfall was at least 20% below average over the southern and western fringes of NSW (CSIRO 2008a). It is uncertain whether this trend is outside natural climatic variation and directly attributable to global warming; however, it could reflect an early indication of changes which may continue and intensify through future decades.

Until 2050 summer rainfall is projected to slightly increase in the north-east of the state, although this will be accompanied by a significant decrease in winter rainfall in the south-western regions (Map 2.2) (DECCW in prep.). Most of NSW is expected to experience a shift from winter-dominated to summer-dominated rainfall. This will have implications for the length and severity of drought in these areas.

Drier conditions across southern Australia (south of 25° latitude) are consistently projected (IPCC 2007a). Changes in the north, and thus influences on tropical rainfall that will potentially affect summer rainfall in northern NSW, are less well understood. Any future variations in rainfall will have implications for water availability and activities, such as farming.

The impacts of climate change on flood-producing rain are expected to be different from the impacts on less extreme rainfall, with the intensity of significant floods 'likely' (>66% probability) to increase even where average rainfalls are expected to decrease (DECCW in prep.). This will tend to increase runoff, erosion and flood risk. This is based on rainfall projections in the Climate Change in NSW Catchments series completed by CSIRO for the NSW Government in 2007 (CSIRO 2007).

Map 2.2: Projected changes in rainfall to 2050

Evaporation

Evaporation is expected to increase significantly across much of the state by 2050 as a result of projected higher temperatures. Summer evaporation is 'likely' (>66% probability) to increase across the state, particularly in central areas of NSW. The potential increases in evaporation are 'likely' to offset the expected increases in summer rainfall, with drier soil conditions expected across the west. The projected drying of the autumn, winter and spring seasons in the south and south-west is expected to be outside the variability observed in historical records (DECCW in prep.).

Sea level rise

Increasing global temperatures have a direct impact on sea levels. As atmospheric temperature increases, so too do ocean temperatures. Because water expands when its temperature rises, any long-term increase in global warming will lead to a corresponding increase in sea levels. Sea level rise can also be expected from melting glaciers and ice caps (IPCC 2007a; DECCW in prep.). Recent evidence suggests that this influence has increased significantly over the past decade, leading to a growing scientific view that future sea level rise is likely to be at the top of the range of the IPCC projections (Cazenave et al. 2009).

Since the late 19th century, global sea levels have risen by 195 mm at an average rate of 1.7 ± 0.3 mm per year (Church & White 2006). The rate of sea level rise accelerated over this period and was estimated at 3.4 mm in 2007 (Beckley et al. 2007). Sea level rise will not be the same across the Earth's surface due to uneven oceanic heating, changes in the mean zones of atmospheric pressure and ocean circulation, and regional geology.

The IPCC projected that global sea levels will rise by 0.18–0.79 m by the end of the 21st century, acknowledging that higher values cannot be excluded and that sea level rise will vary regionally (IPCC 2007a). The NSW Government has adopted sea level rise benchmarks of 0.4 m by 2050 and 0.9 m by 2100 relative to 1990 sea levels for planning purposes (DECCW 2009a).

The benchmarks comprise the upper limit of the IPCC sea level rise projections – 0.3 m by 2050 and 0.79 m by 2100 – and CSIRO projections for a regional variation from the global average of 0.1 m by 2050 and 0.14 m by 2100 (DECCW 2009b). In adopting these benchmarks the NSW Government has considered three important observations:

• Global greenhouse gas emissions have risen rapidly since the year 2000 and are exceeding the IPCC emissions projections (Steffen 2009).
• Sea level rise has accelerated and is exceeding the IPCC projections (Church & White 2006; Rahmstorf et al. 2007).
• Sea levels off the NSW coast have risen faster than the global average and are expected to increase more than the global average projections (IPCC 2007a; McInnes et al. 2007).

It is this last consideration that sets NSW apart from other Australian states, with the CSIRO projections for the NSW coastline being higher than for any other part of the Australian coast (DECCW 2009b).

Pressures

There is a consensus among climate scientists that the climate change NSW is experiencing is 'very likely' (>90% probability) the result of increased greenhouse gas emissions from human activities (CSIRO & BoM 2007). See Climate Change 2.2 for the source of greenhouse gas emissions in NSW.

Responses

The NSW Government is contributing to global efforts to reduce greenhouse gas emissions to help limit the rate and magnitude of climate change. Responses for both this section and the next are discussed in Climate Change 2.2. NSW Government actions to help the state adapt to the unavoidable impacts of climate change are discussed in Climate Change 2.3.

Future directions

It is expected that there will be an increase in temperature and sea levels, and changes to rainfall patterns and evaporation rates in NSW due to anthropogenic greenhouse gas emissions. The fourth IPCC assessment report states that effective mitigation and adaptation strategies must be implemented immediately to reduce the risks that climate change poses to the state's environmental, social and economic systems. It has been suggested that even minor delays in implementing mitigation strategies could result in an escalation of the severity of climate change impacts for centuries to come (Parry et al. 2009).

The NSW Government is developing the Climate Change Action Plan which will define its future role in light of the Australian Government's commitments to greenhouse gas emission reduction and the availability of more specific scientific information. Some of the most challenging aspects of preparing for the unavoidable impacts of climate change will be connected with rising sea levels, the reduction of water availability in the south-west of the state and the reduction of snow cover in the Australian Alps.

Scientific research will need to be prioritised to focus on the uncertainties that still remain in the field of climate science. For example, the relationship between the El Niño – Southern Oscillation (ENSO) and climate change is unclear. Further research is required to explore the links between climate change and ENSO and other climate influences, as this research is significant in order to develop climate change projections for NSW. Recent data suggests that the significance of the interaction between drought and the Indian Ocean Dipole may be greater than previously realised and needs further examination (Ummenhofer et al. 2009).