Many climate change impacts can be projected with confidence, but for others, the scientific understanding is still evolving. This is particularly true for processes that have no precedent in recorded history, such as when warming pushes parts of the climate system past critical thresholds that can lead to irreversible changes.
Scientists know that human activities are behind the elevated concentrations of carbon dioxide in our atmosphere. This is confirmed by measuring the forms (isotopes) of carbon in the atmosphere that reveal their source, namely fossil fuels and land use changes.
Climate change threatens all nations and peoples. But it disproportionately affects the poorest, including the world’s most vulnerable: indigenous peoples, women, children and the elderly. They will be the first to lose their homes as sea levels rise, struggle to find work or health care and face food insecurity in the wake of droughts and floods. The richest countries, which produce the most greenhouse gases, must take the lead in cutting emissions and providing financial support to help the poorest adapt.
Even with aggressive action, it’s highly likely that we will overshoot global temperature targets laid out in the Paris Agreement—including a goal to limit the increase to 1.5 degrees Celsius. As we warm the Earth, oceans will continue to expand and glaciers and ice sheets melt, raising sea levels for centuries. And as the Arctic permafrost thaws, it releases carbon that was stored in the ground as solid ice—reinforcing the effect of warming and accelerating its pace.