Thursday 10 July 2008
The Royal Society of New Zealand convenes an expert committee on climate. The controversy over climate change and its causes, and possible confusion among the public, have prompted the committee to produce a statement to make absolutely clear what the evidence is for climate change and anthropogenic (human-induced) causes.
The globe is warming because of increasing greenhouse gas emissions. Measurements show that greenhouse gas concentrations in the atmosphere are well above levels seen for many thousands of years. Further global climate changes are predicted, with impacts expected to become more costly as time progresses. Reducing future impacts of climate change will require substantial reductions of greenhouse gas emissions.
The Royal Society is charged by its Act with informing the public about science, and fostering evidence-based scientific debate. We hope this statement makes a useful contribution to public understanding of climate change.
The globe is warming, because of increasing greenhouse gas emissions.
There has been an overall upward trend in global surface temperature since the beginning of the 20th Century. Most of the observed global warming over the past 50 years is very likely to be due to increases in greenhouse gas concentrations in the atmosphere. Greenhouse gases warm the lower atmosphere by allowing sunlight to reach the Earth’s surface but trapping some of the infrared radiation emitted by the Earth. Human activities have increased the concentrations of carbon dioxide, methane and nitrous oxide since the mid-1700s. More than half of the carbon dioxide concentration increase has occurred since 1970.
Human activities have also increased concentrations of aerosols (small “air pollution” particles) in the atmosphere. These may have partially offset the heating effect of the greenhouse gases by scattering some sunlight back to space.
Natural factors also cause climate variations.
Climate has always varied, over timescales of decades, centuries and millennia. Until recently these variations have had only natural causes – including changes in the tilt of the Earth’s axis, the shape of the Earth’s orbit, the energy output from the sun, dust from volcanic emissions, and heat exchanges between the atmosphere and the ocean (such as El Niño). This natural variability still occurs in addition to the human influences. Thus while the overall decade-to-century temperature trend is upwards, individual years can still be warmer or cooler than previous years.
Further global changes are predicted. Many impacts are expected to be more costly as time progresses.
Even if the concentrations of all greenhouse gases and aerosols were held constant at year 2000 levels, a further warming trend would be expected for at least several decades, due mainly to the slow response of the oceans. Additional increases in greenhouse gas concentrations, and resulting changes in climate, will occur over coming decades unless concerted international action is taken to substantially reduce emissions. Impacts will vary regionally but, aggregated and discounted to the present, they are very likely to impose net annual economic costs which will increase over time as global temperatures increase.
- Atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased by 35%, 150% and 18% respectively since around 17501 .
- Air temperature (averaged over the globe’s surface) has risen through the past 100 years. The linear warming trend from 1906 to 20052 was 0.74°C [0.56 to 0.92°C]3.
- Globally-averaged sea level rose by 17 cm [12 to 22 cm] during the 20th century.
- Average northern hemisphere temperatures during the second half of the 20th century were very likely warmer than during any other 50-year period in the last 500 years and likely the warmest in the past 1300 years.
- For the globally-averaged surface air temperature, 2005 and 1998 were the two warmest years in the instrumental temperature record (i.e. since 1850. Twelve of the thirteen years during the period 1995-2007 are the warmest since 1850).
- Mountain glaciers and snow cover have declined on average in both hemispheres.
- Spring peak river flows have been occurring 1 to 2 weeks earlier in basins with important seasonal snow cover in North America and northern Eurasia (based on observations over the period 1936 – 2000), due to earlier warming-driven snow melt.
- Arctic sea-ice summer extent has decreased at an average rate of 7.4% [5.0% to 9.8%] per decade since 1978.
- Observations since 1961 show the average temperature of the global ocean has increased to depths of at least 3000m, with decadal fluctuations superimposed on this long-term trend.
- The ocean has become more acidic because of uptake of carbon dioxide from the atmosphere.
- For New Zealand the air temperature shows substantial year to year fluctuations, but shows a clear increase over time, with a linear trend in the country-wide average of 0.9°C between 1908 and 2006. The average sea level rise over the twentieth century was 16±2 cm. The number of frost days has decreased since the 1950s at many New Zealand sites.
- Present global atmospheric concentrations of carbon dioxide far exceed pre-industrial values dating back at least 650,000 years.
- The global increases in carbon dioxide concentration since pre-industrial times are due primarily to fossil fuel use and land use change.
- About 2/3rds of these anthropogenic carbon dioxide emissions since 1750 are estimated to have come from fossil fuel burning and about 1/3 from land use change. About 45% of this carbon dioxide has remained in the atmosphere.
- The combined influence on the energy balance of the Earth-atmosphere system of all anthropogenic emissions between 1750 and 2006 is likely to be at least 5 times larger than the influence of changes in solar output over the same period.
- Very energetic volcanic eruptions (such as that of Mt Pinatubo in 1991) can place small particles high in the atmosphere, blocking sunlight and leading to cooling for a few years.
- Increasing atmospheric temperatures lead to an overall increase of water vapour in the atmosphere. Water vapour is itself a strong greenhouse gas, so this amplifies the warming effect of the increase in anthropogenic greenhouse gases.
- The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour.
Projections for the 21st Century from the IPCC’s4 2007 assessment (assuming no substantial efforts to reduce greenhouse gas emissions) include:
- An increase in globally-averaged surface temperature of 1.1 to 6.4°C by 21005.
- A globally-averaged sea level increase of 18 to 59 cm by 2100. However these projections do not include uncertainties in climate-carbon cycle feedbacks nor the full effects of dynamic changes in ice-sheet flow, so do not provide an upper bound for possible sea level rise.
- More heat waves, fewer frosts, and more heavy rain events are very likely.
- The area affected by droughts is likely to increase through the 21st Century.
- Increases are likely in the peak wind and rain intensity in tropical cyclones
- A New Zealand-average warming7 of 0.2 to 2.0°C by 2040 and 0.7 to 5.1°C by 2090. Fewer cold temperatures and frosts, and more high temperature episodes.
- A stronger west-east rainfall gradient (wetter in the west and drier in the east) in winter and spring, and an increasing risk of extreme rainfall as the century progresses.
- Increasing drought risk during this century in areas which are currently drought-prone.
- An increase in New Zealand-averaged sea level of the same order as the IPCC global projections.
- Natural year to year variations in New Zealand’s climate will be superimposed on top of these projected anthropogenic changes.
- Some studies suggest substantial parts of the Greenland ice cap, and perhaps of the West Antarctic ice sheet could melt over the coming 1000 years. Global average sea level at the height of the last interglacial about 125,000 years ago (when average polar temperatures were around 3°C to 5°C warmer than now) was likely 4-6 m higher than at present.
- Models indicate some slowing of the oceanic meridional overturning circulation (MOC) is very likely during the 21st Century. The MOC carries warm water into the North Atlantic where it cools, sinks and then spreads through the other ocean basins at depth
- As the globe warms, the oceans and biosphere may become less efficient at absorbing carbon dioxide, leading to a larger fraction of the anthropogenic emissions remaining in the atmosphere.
Reducing the future impact of climate change will require substantial reductions of net emissions of greenhouse gases. Major international policy changes would be required to deliver these reductions but various technologies exist to provide them:
- More efficient use of energy, e.g. better designed and insulated houses, more efficient appliances and industrial processes
- Renewable energy sources, e.g. hydropower, geothermal, wind, marine, and solar
- Lower-carbon fossil fuels such as natural gas and the capture and storage of emissions from power plants
- More efficient transport and urban systems and the use of appropriate biofuels and other renewable energy sources to power transport
- Reforestation, reduced deforestation, and lower emission forms of agriculture
1. These percentage increases are based on greenhouse gas concentrations in 2005 documented in the IPCC’s Fourth Assessment Report.
2. This is the best straight-line fit to the time series of annual global temperatures over this time interval.
3. Values provided in square brackets are estimated 90% uncertainty intervals, ie there is an estimated 5% likelihood that the value could be below the lowest number provided, and 5% likelihood it could be above the highest value.
4. IPCC = Intergovernmental Panel on Climate Change.
5. Part of the range for these temperature projections reflects the range of plausible greenhouse gas emission trajectories for the coming century, depending on economic, social and policy developments. Part of the range is from uncertainties in global climate models. The changes quoted here are relative to the period 1980-99.
6. Note that projections of regional changes in climate are subject to more uncertainty than projections of global changes.
7. These projected changes are relative to the period 1980 – 1999.