Will reducing atmospheric CO₂ reverse the effects of climate change over a few decades?

Question

There are many calls to help bring down Carbon Dioxide levels. But what are the effects of such an action?

My intuition of the earth as a slow-moving complex system tells me that it will settle into a "new state" in which new patterns of temperature and rain will establish for a very long time.

But a widespread opinion says that we will able to reverse the rise in temperatures and other effects like rainfall patterns if we reduce emissions. It seems a simpler explanation to sell and to believe in (most people, like me, are not trained in complex systems theory!).

These emissions change the balance of gases in the atmosphere, and cause climate change, global warming and degrade the ozone layer. It up to all of us to reduce our emissions to reverse these effects.

HowStuffWorks says thus:

Assuming that the IPCC's conclusions are accurate, reducing greenhouse gas emissions and planting trees could help slow and eventually reverse global warming trends.

A call to reduce emissions will get more support if we promise to get it reversed in decades, rather than ambiguously reduce the stimulus that is accelerating changing climate patterns.

And so it is a common-enough notion. But is it true?

Answer 1

Without geo-engineering solutions, a certain amount of warming is irreversible. Even if we stop emitting CO₂ today, the energy is in the system and it will cause a certain amount of warming.

Dr. James Hansen of NASA Goddard has talked a lot about this. He often says that there is already warming "in the pipeline" that is irreversible.

In a 2005 paper in Science Magazine he says:

Earth's energy imbalance. We infer from the consistency of observed and modeled planetary energy gains that the forcing still driving climate change, i.e., the forcing not yet responded to, averaged ∼0.75 W/m² in the past decade and was ∼0.85 ± 0.15 W/m² in 2003. This imbalance is consistent with the total forcing of ∼1.8 W/m² relative to that in 1880 and climate sensitivity of ∼2/3°C per W/m². The observed 1880 to 2003 global warming is 0.6° to 0.7°C, which is the full response to nearly 1 W/m2 of forcing. Of the 1.8 W/m² forcing, 0.85 W/m² remains, i.e., additional global warming of 0.85 × 0.67 ∼ 0.6°C is “in the pipeline” and will occur in the future even if atmospheric composition and other climate forcings remain fixed at today's values.

from Earth's Energy Imbalance: Confirmation and Implications (DOI: 10.1126/science.1110252)

There is inertia in the Earth System. The CO₂ has been emitted, and the radiation balance is off. It will take a while for that warming to show up in the climate, but it will happen. The Earth System is out of equilibrium and is working towards getting to it, and it will be a warmer equilibrium.

However, this doesn't mean we can just forget about trying to reduce CO₂ emissions because we're screwed anyway. This image from the IPCC AR4 shows the model predictions of warming for different CO₂ emission scenarios. The yellow line represents if we could manage to get our atmospheric CO₂ concentrations down to 2000 levels, and the red line represents a business as usual approach where we make no changes whatsoever.

IPCC Fourth Assessment Report, Working Group I: The Physical Science Basis

Projections of Future Change in Climate

You can see from this chart that the scenario we choose, and how much CO₂ that we emit in the future, does make a difference in considering how much warming will occur. Reducing CO₂ emissions is crucial for being able to mitigate the amount of warming that will occur, but at this point we've already locked in a certain amount of warming becuase of past CO₂ emissions.