Is it too late to reverse climate change?

Is it too late to reverse climate change? I'll check the latest science on what's still reversible and what isn't.

No, it is not too late to limit and partially reverse climate change - but it is too late to fully reverse it quickly, and some changes are already irreversible on human timescales.

1. Some warming is already irreversible

Scientific assessments agree that:

> “A large fraction of climate change is largely irreversible on human time scales,” the most recent assessment report from the Intergovernmental Panel on Climate Change (IPCC) warned. Only if human emissions were “strongly negative over a sustained period”... would climate change begin to be reversed

A NOAA-led study reached a similar conclusion:

> Climate Change Largely Irreversible For Next 1,000 Years, NOAA Reports

NASA summarizes it this way:

> Humans have caused major climate changes to happen already, and we have set in motion more changes still

> While the effects of human activities on Earth's climate to date are irreversible on the timescale of humans alive today, every little bit of avoided future temperature increases results in less warming that would otherwise persist for essentially forever

What is considered effectively irreversible includes:

Some climate-related damage will be irreversible, and some could be partially reversed with significant delay

2. We can still stop further warming relatively quickly

Reversing is different from stopping.

> However, if we stopped emitting greenhouse gases today, the rise in global temperatures would begin to flatten within a few years. Temperatures would then plateau but remain well-elevated for many, many centuries. There is a time lag between what we do and when we feel it, but that lag is less than a decade

Recent research has narrowed that lag even further to "more like three to five years" because oceans take up carbon more rapidly when emissions stop, which "mostly" offsets the remaining warming effect

NOAA Climate.gov explains the same mechanism:

> If all human emissions of heat-trapping gases were to stop today, Earth's temperature would continue to rise for a few decades as ocean currents bring excess heat stored in the deep ocean back to the surface. Once this excess heat radiated out to space, Earth's temperature would stabilize

> With no further human influence, natural processes would begin to slowly remove the excess carbon dioxide from the atmosphere, and global temperatures would gradually begin to decline

Without human intervention, that natural decline is very slow:

> And over several thousand years, Earth would dial temperatures back down to preindustrial levels via natural carbon sinks, such as trees and soils absorbing CO2, according to NOAA

This also applies if emissions continue a while longer:

> This means that even if carbon emissions dropped to zero today, global temperatures would continue to rise for a few decades, with experts predicting an extra 0.5 C (0.9 F) of warming from oceans alone

3. True reversal to 1.5°C requires net-negative emissions

To actually bring temperatures down after overshoot, we would need to remove more CO2 than we emit.

Estimates suggest up to 400 gigatonnes of carbon dioxide would need to be removed from the atmosphere by 2100 to limit warming to 1.5°C under a higher climate sensitivity, which is equivalent to running the US energy industry in reverse for around 80 years.

> Even assuming that removing that amount of carbon is feasible, some Earth systems will probably not be restored to their previous equilibrium

4. It is not too late to avoid the worst outcomes

Both NASA and NOAA stress that action now still matters greatly:

> But it may not be too late to avoid or limit some of the worst effects of climate change

> it is never "too late" to take action

> While we cannot stop global warming overnight, we can slow the rate and limit the amount of global warming by reducing human emissions of heat-trapping gases and soot

NASA describes the needed response as a two-tier approach:

> - "Mitigation" - reducing the flow of greenhouse gases into the atmosphere
> - "Adaptation" - learning to live with, and adapt to, the climate change that has already been set in motion

The IPCC's Sixth Assessment Report said to keep 1.5°C within reach:

> Emissions must peak by 2025 to limit global warming and reduce by 43% by 2030

Without major action, global temperature is on track to rise by 2.5°C to 4.5°C by 2100

Bottom line

In short, past warming cannot be quickly undone, but future warming is still a choice. The sooner emissions peak and fall, the less irreversible damage is locked in.