Decisions made by governments early in this century will have long-term consequences for the climate.
Failure to take meaningful actions to reduce global emissions is a particularly serious decision. By continuing on its current path of increasing global carbon dioxide emissions, humanity is committing future generations to a strongly altered climate. Even beyond the current century, there are major implications for longer-term climate change.
Higher temperatures and accompanying changes in climate caused by carbon dioxide emissions from human activity will be largely irreversible on human time scales. The effects are long-lasting. Atmospheric temperatures are not expected to begin to decrease substantially for many centuries to millennia, even after human-induced greenhouse gas emissions stop completely.
To meet the 2 degree Celsius target, the climate system itself thus imposes a timescale on when emissions need to peak and then begin to decline rapidly.
Scientific understanding of this timescale shows that mitigation of climate change is urgent and cannot wait. A window of opportunity to decrease emissions has been open for several decades but will soon close and will remain closed. This urgency is thus not ideological or political, but rather is due to the long residence time of atmospheric carbon dioxide and the resulting near-irreversibility of climate change for centuries or longer.
This is a critical shortcoming of the medical metaphor that likens the climate change problem to the task of an individual trying to reach a target body weight. If your current weight is higher than your target, medical science can advise you on how to lose weight. If the amount of carbon dioxide in the atmosphere becomes higher than the amount compatible with the desired limit on global warming, however, the warming will exceed the target.
There is no proven technology for removing large amounts of carbon dioxide from the atmosphere economically.
In short, the urgency of acting to mitigate climate change is directly linked to the physics and biogeochemistry of the climate system. An enduring failure to achieve meaningful science-based international agreements to rapidly reduce emissions of heat-trapping gases will inevitably have serious consequences for the degree of climate change that the Earth will undergo.
If the world as a whole continues to procrastinate throughout the current decade, allowing emissions to continue to increase year after year, then it will almost certainly have lost the opportunity to limit warming to 2 degrees Celsius.
Instead, our children and their descendants, and ultimately all living things, will be faced with the consequences of more severe climate disruption.