A distinction needs to be made between science that is robust and science that is relatively uncertain. All conclusions should be based on peer-reviewed literature.
Science is rarely, if ever, 'settled'. The nature of the scientific method – whereby hypotheses are routinely questioned, tested, refined and retested – is such that understanding is improved over time.
The peer-review process is central to this by minimising the chance of low quality science being published – and eventually self-corrects any that does. Better questions to consider are 'Is the science robust?' and 'What are the reasons for uncertainties and can they be reduced?'
In climate change science, the robust findings include:
- clear evidence for global warming and sea level rise over the past century
- changes observed in many physical and biological systems are consistent with warming
- due to the uptake of anthropogenic CO2 since 1750, ocean acidity has increased
- most of the global average warming over the past 50 years is extremely likely due to anthropogenic greenhouse gas increases
- 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 opportunities to achieve lower greenhouse gas stabilisation levels.
Some of the key uncertainties include:
- observed climate data coverage remains limited 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
- effects of climate changes on human and some natural systems are difficult to detect 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 feedbacks in the climate system, particularly cloud feedbacks, oceanic heat uptake and carbon cycle feedbacks
- 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 the magnitude of the temperature response, on clouds and on 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
- understanding of low-probability/high-impact events and the cumulative impacts of sequences of smaller events is generally limited
- barriers, limits and costs of adaptation are not fully understood
- estimates of mitigation costs and potentials depend on 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 the uncertainties and quantify confidence levels.