Science is rarely, if ever, 'settled'. Scientific findings rely on a stream of evidence, with hypotheses (tentative explanations) routinely questioned, tested, refined and retested. This is how our understanding improves over time.
The peer-review process is central to improving our scientific understanding. It involves scientists (‘peers’) evaluating other scientists’ work. The aim is to ensure that the work is robust – rigorous, logical, uses past research and adds to what we already knew.
In climate change science, the robust findings include:
- continued increases in atmospheric concentrations of major greenhouse gases, such as carbon dioxide and methane
- clear evidence for temperature and sea level rise since the 1800s
- changes observed in many physical and biological systems are consistent with warming
- increases in ocean acidity due to the uptake of human-released carbon dioxide
- human activities are estimated to have caused approximately 1 °C of global warming above pre-industrial levels
- continued global greenhouse gas emissions will lead to further climate change
- due to the time scales associated with climate processes and feedbacks, anthropogenic warming and sea level rise would continue for centuries even if greenhouse gas emissions were to be reduced sufficiently for atmospheric concentrations to stabilise
- increased frequencies and intensities of some extreme weather events are very likely
- systems and sectors at greatest risk are ecosystems, low-lying coasts, water resources in some regions, tropical agriculture, and health in areas with low adaptive capacity
- the regions at greatest risk are the Arctic, Africa, small islands and Asian and African mega-deltas. Within other regions (even regions with high incomes) some people, areas and activities can be particularly at risk
- some adaptation is underway, but more extensive adaptation is required to reduce vulnerability to climate change
- unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt
- many impacts can be reduced, delayed or avoided by mitigation (net emission reductions). Mitigation efforts and investments over the next two to three decades will have a large impact on achieving lower greenhouse gas stabilisation levels.
Some of the key uncertainties in climate change science include:
- limited climate data in some regions
- analysing and monitoring changes in extreme events is more difficult than for climatic averages because longer data sets with finer spatial and temporal resolutions are required
- understanding of low-probability/high-impact events and the cumulative impacts of sequences of smaller events is generally limited
- effects of climate changes on human and some natural systems are difficult to determine due to adaptation and non-climatic influences
- difficulties remain in reliably attributing observed temperature changes to natural or human causes at smaller than continental scales
- models differ in their estimates of the strength of different climate system feedbacks, particularly those relating to clouds, oceanic heat uptake and the carbon cycle
- confidence in projections is higher for some variables (e.g. temperature) than for others (e.g. rainfall), and it is higher for broad-scale and long-term changes
- direct and indirect impacts of aerosol (fine atmospheric particles) on atmospheric temperature , clouds and rainfall remain uncertain
- future changes in the Greenland and Antarctic ice sheet mass are a major source of uncertainty that could increase sea level rise projections
- impact assessment is hampered by uncertainties surrounding regional projections of climate change, particularly rainfall
- barriers, limits and costs of adaptation are not fully understood
- estimates of mitigation costs and potential depend on inherently uncertain assumptions about future socio-economic growth, technological change and consumption patterns.
A sound basis for action
There is ample, well-supported evidence to provide a basis for action through mitigation of greenhouse gas emissions and for adaptation to reduce our vulnerability to climate change impacts.
At the same time, further research is needed to reduce uncertainties and quantify and improve confidence levels.