What is climate change?

Knowing the climate science can help in better interpreting and understanding the role of climate change projections in planning. This section explores the key definitions used in understanding climate change and describes the climate trends in Victoria as well as the climate change projections at a regional scale.

For the South West region, CSIRO and the Bureau of Meteorology produced a technical climate report, available at: www.climatechangeinaustralia.gov.au/en/publications-library/cluster-reports

Information for this section has also been sourced from Southern Slopes Information Portal Report: Climate change adaptation information for natural resource planning and implementation developed by SCARP. The full report can be found at: www.climatechangeinaustralia.gov.au/en/impacts-and-adaptation/southern-slopes

In order to understand how climate change affects us at local, catchment and/or regional scale, we need to first understand what climate change is and how it is affecting us at a global scale. 

Climate change is often described as the change in the average weather over a long period of time, usually over a period of 30 years. Climate change can occur due to a combination of natural and human causes. The United Nations Framework Convention on Climate Change (UNFCCC, 1992) describes climate change as:

...a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.

Over geological timescales, even rapid climatic change occurred much slower than the current rate of change. For example, it took centuries for the ice from the last glaciation to decline (about 10,000 years ago). By comparison, in the last century the global climate has changed rapidly, with an increase of around 0.74°C each year (1906-2005). Most of this change occurred during the second half of the last century, with eleven of the twelve warmest years occurring between 1995 and 2006. 2015 was the hottest year on record, the previous hottest year was 2014. (Climate Change Council, 2015). In Australia, there has been a warming of 0.9°C each year since 1910 (CSIRO and Australian Bureau of Meteorology, 2014). 

Atmospheric concentrations of greenhouse gases (CO2, methane, and nitrous oxide) are now at the highest level they have been for at least the last 800,000 years (CSIRO and Australian Bureau of Meteorology, 2014). Concentrations of CO2 have increased by 40% since preindustrial times, due mainly to emissions from fossil fuels and changes to land use. In its latest report, the IPCC (IPCC, 2013a) states: “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century”.

What is climate variability?

Climate variability is defined as ”variations in the mean state of the climate and other statistics (such as standard deviations and the occurrence of extremes) across temporal and spatial scales beyond that of individual weather events” (IPCC, 2007). It is important for climate variability not to be confused with climate change. Typically, climate variability relates to shorter periods of time than climate change.

Weather is usually associated with day-by-day atmospheric change, whereas climate change refers to changes occurring over decades or centuries. For example, intense and heavy rainfall causing consecutive years of flood events can be described as climate variability, whereas an increase in rainfall over several decades is referred to as climate change. A single heavy rainfall event is part of daily weather.

Why is climate variability important for adaptation?

Climate variability can result in extreme weather events, such as droughts, heavy rainfall, fire weather, heat waves, hail storms and flooding. These extreme weather events can occur in increased frequency and intensity over a period of several weeks, months or years. They have strong and immediate impact on human lives, assets and natural resources.

Climate variability, especially extremes, is a key short to medium term consideration for climate change adaptation. For example, the region’s farmers may consider climate variability in their planning to manage seasonal climate differences – maximising profit in good seasons and managing through poor seasons. However, climate change adaptation can be more strategic and encourages a long-term approach. Long-term change such as decreasing average rainfall or increased summer temperatures are taken into account. Farmers may think strategically about shifting some of their practices and sources of revenue to accommodate any medium and long term changes to the climate, or diversify their livelihoods to be less reliant on rainfall that experience and projections might indicate is becoming less reliable.

What are climate change projections?

Climate change projections are scientific statements, based on the output of global or regional climate models, about changes in aspects of the future climate. At a global scale, there are substantial uncertainties regarding what the actual climate change projections will be. These uncertainties are represented in climate models as emissions scenarios. These essentially simplify assumptions about climate diplomacy, technological change and development trajectories into scenarios of atmospheric models associated with greenhouse gas concentrations (measure in parts per million [ppm] of CO2 equivalents).

Scenarios are referred to by the IPCC as Representative Concentration Pathways (RCPs). The highest RCP (RCP 8.5) assumes a concentration of 1350 ppm by 2100. Projected global mean temperatures associated with this scenario range from 2.6-4.8°C above current temperatures. A mid-range scenario assumes 650 ppm (RCP 4.5), projected increases range from 1.0-2.6°C. Currently, global emissions have consistently tracked at or above the highest emissions scenario (RCP 8.5). The projections for the Southern Slopes Region state:

For the near future (2030), the projected increase of mean annual temperature is around 0.4 to 1.1 °C above the climate of 1986–2005, with only minor differences between RCPs. For late in the century (2090), there is a large difference between scenarios, with projected warming of 1.1 to 2.0 °C for RCP4.5 and 2.5 to 4.0 °C for RCP8.5.

Grose et al., 2015

Climate projections are dependent on a set of influential conditions, such as changes in atmospheric greenhouse gases. Because of these dependencies on external conditions, projections are not projections of the future, but only an expression of a conditional expectation. That is, they model the changes in atmospheric and oceanic circulation and conditions given differing fundamental conditions such as greenhouse gas concentrations, particles (aerosols) in the atmosphere, or changes in solar activity.

It is not possible to project the future climate, because of the uncertainty around future emissions, the uncertainty represented by the range in climate models, and the natural variability of the climate system. The intention of simulating future climate is therefore not to make accurate projections regarding the future state of the climate system at any given point in time. Rather it is to provide model-derived descriptions of possible future climates under a given set of scenarios of climate models (IPCC, 2007).

The effect of climate change on key climate variables across south-eastern Australia is relatively well understood in general terms. Differences in the direction and magnitude of change are increasingly being resolved at the regional level. In contrast, the regional impacts associated with these changes are less well known. Table 1 provides an overview of potential impacts of changes to the key climate variables.

Table 1: Climatic variables of most impact across the south west region’s natural asset types

  Key Climate Variable Associated Indices Native
Waterways Estuaries Wetlands Coastal Wetlands Flora & Fauna Soil
Air Temperature Average Annual Temperature              
Minimum Daily Air Temperature              
Maximum Daily Air Temperature              
Seasonal Temperature Regimes              
Snow Cover                
Sea Surface Temperature              
Marine Heat Waves                
Drought/Extended Dry Spells              
Rainfall Seasonal Rainfall (decrease)              
Regional Runoff (decrease)              
Seasonal Runoff (decrease)              
Riverflow (decrease)              
Intense Rainfall (increase)              
Bushfire Intensity (increase)              
Bushfire Frequency (increase)              
Sea Level Incremental SLR                
Extreme sea-level/ storm surges/tide              
Atmospheric CO2 Concentrations Atmospheric CO2                
Ocean acidification              
Wind & Extreme Wind Events Annual Average Wind Speed & Direction              
Extreme Wind                
Total Cloud Cover                
Radiation & Evaporation Average Annual Radiation              
Humidity Annual Average Relative Humidity              

Key: (red = high impact; orange = medium impact; yellow = low impact)