Tuesday, March 22, 2016

How Can Global CO2 Levels Soar When Emissions Are Flat? - by Joe Romm

Bathtub Earth (Credit: EPA) Click to Enlarge.
Last year saw the biggest jump in global CO2 levels ever measured, as NOAA reported on March 9.  Yet in 2015 the world economy grew while energy-related CO2 emissions were flat — for the second year in a row — according to the International Energy Agency, as ClimateProgress reported last week.
What’s going on?  Two things
  • Annual CO2 emissions are very different from global CO2 levels.
  • CO2 levels tend to have big jumps in El NiƱo years.
Let’s go through those two, especially since this discussion gets to the heart of what I call “the biggest source of confusion in the public climate discussion” in my recent book, Climate Change: What Everyone Needs to Know.  It also goes to the heart of why delaying action is so dangerous and costly.

The CO2 Bathtub Analogy
Avoiding catastrophic warming requires stabilizing CO2 concentrations (or levels) in the atmosphere, not annual emissions.  Studies find that many, if not most, people are confused about this, including highly informed people, mistakenly believing that if we stop increasing emissions, then global warming will stop.  In fact, very deep reductions in greenhouse gas emissions are needed to stop global warming.

One study by MIT grad students found that “most subjects believe atmospheric GHG concentrations can be stabilized while emissions into the atmosphere continuously exceed the removal of GHGs from it.”  The author, Dr. John Sterman from MIT’s Sloan School of Management, notes that these beliefs “support wait-and-see policies but violate conservation of matter” and are “analogous to arguing a bathtub filled faster than it drains will never overflow.”

While atmospheric concentrations (the total stock of CO2 already in the air) might be thought of as the water level in the bathtub, emissions (the yearly new flow into the air) are the rate of water flowing into a bathtub from the faucet.  There is also a bathtub drain, which is analogous to the so-called carbon “sinks” such as the oceans and the soils.  The water level won’t drop until the flow through the faucet is less than the flow through the drain.

Similarly, carbon dioxide levels won’t stabilize until human-caused emissions are so low that the carbon sinks can essentially absorb them all.  Under many scenarios, that requires more than an 80 percent drop in CO2 emissions.  And if the goal is stabilization of temperature near or below the 2°C (3.6 °F) threshold for dangerous climate change that scientists and governments have identified, then CO2 emissions need to approach zero by 2100.

So the first key point is that CO2 levels will continue rising if we merely keep annual CO2 emissions flat.  In fact, they will keep rising at a faster and faster rate because the land and ocean carbon sinks are weakening.

Read more at How Can Global CO2 Levels Soar When Emissions Are Flat?

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