- We have clear evidence of human-caused climate change.
- The Earth is warming rapidly. Both land and sea temperatures have increased over the past 100 years, and are increasing more and more rapidly.
- Other changes and evidence of climate change are being observed. These include a reduction in the mass of global ice caps and glaciers, rising sea levels, acidification of our oceans, and increased frequency and intensity of extreme weather events.
- These changes are expected to continue, even if we reduce our greenhouse gas emissions, because of existing emissions and changes to our climate systems.
Evidence of global warming
The Intergovernmental Panel on Climate Change (IPCC) has said that ‘it is unequivocal that human influence has warmed the atmosphere, ocean and land'.
The planet's average surface temperature rose about 1.18 °C from the late 1800s to 2020. Most of the warming has occurred in the past 40 years, and each decade since 1980 has been warmer than the decade before. The years 2016 and 2020 are tied for the warmest year on record globally.
Our oceans have also warmed. The top 100 metres of ocean has warmed more than 0.33 °C since 1969, and the average sea-surface temperature in 2019 was second only to 2016, which was a record El Niño year.
Evidence of other changes
Climate change does not just mean higher land and ocean temperatures. We can already see how climate change is affecting many aspects of our climate and environmental systems.
Reduced ice and snow
The Greenland and Antarctic ice sheets are melting and have decreased in mass. In 2019, the Antarctic ice sheet had a record low monthly mean sea ice extent. Also, mountain glaciers and snow cover have declined throughout the world.
Rising sea levels
Sea levels are rising because the ocean expands as it warms, and because melting ice sheets and glaciers are adding more water to our oceans. Overall sea levels have risen about 20 centimetres in the past 100 years and the rate of rise is increasing each year.
Some of the increased levels of carbon dioxide in our atmosphere dissolve in the ocean, which makes the water more acidic. Since the early 1800s, the acidity of surface ocean waters has increased by about 30% (0.1 pH units).
Current global outlook
Even if we reduce our greenhouse emissions, global temperatures will continue to rise for decades to come. Global warming of 1.5C and 2C will be exceeded during the 21st century unless deep reductions in greenhouse gas emissions occur in the coming decades. The global ocean will continue to warm, mostly in tropical and subtropical regions. There will be more frequent hot, and fewer cold temperature extremes across most land areas.
The IPCC reports that the net damage costs of climate change are likely to be significant and to increase over time. Adaptation will be needed to reduce the impact of unavoidable warming.
Climate change: how do we know – NASA
Warming of the climate system is unequivocal: highlights of the IPCC Fourth Assessment Report – United Nations
National climate assessment: 21st century temperature scenarios - Scientific Visualization Studio