This was written by Rebecca Anderson, leading up to the UN climate talks in Copenhagen, Denmark on November 19, 2009.
Bottom line: The only significant way that science will contribute at all to Copenhagen is by telling the politicians and negotiators what the absolute, bare minimum is that needs to be done to avoid really big, bad things happening.
This breaks down into two categories:
1. How do you define “dangerous” climate change?
2. What do we need to do, in terms of CO2 emissions and levels in the atmosphere, to avoid it?
So, #1: What is “dangerous” climate change?
The IPCC defines it as risks to a few key categories, shown in this diagram, often referred to as the “burning embers” diagram:
But the key here is that we don’t know exactly how sensitive the climate system is to these things. That’s because there’s uncertainty both in the consequences term of the risk equation as well as the probability term.
RISK = CONSEQUENCES * PROBABILITY
When we look specifically at the risk of getting to a certain temperature (like 2ºC) in the future, this means we have to think about how to translate that to some level of CO2 in the atmosphere. This is a tricky thing to do, because it depends on how sensitive our climate system is that that amount of CO2.
Is it super sensitive? (For the 2x pre-industrial, or 560 ppm, scenario, this means 4.5-7ºC warmer. You can think of this like the Papa Bear scenario – BIG AND BAD.)
Is it moderately sensitive? (Around 3-3.5ºC warmer, akin to Momma Bear.)
Or is it not that sensitive? (Only 2ºC warmer, a Baby Bear scenario, by far the least scary and best of all options.)
Most scientists think it’s the Momma Bear case – about 3.5ºC warming for 2x CO2. The Baby Bear isn’t very likely, but the Papa Bear is a definite possibility.
The reason scientists aren’t sure how sensitive we are is because it’s really hard to pin down when potential tipping points could be crossed and how intensely positive feedbacks (sea ice melt, permafrost melt) can kick in.
The real kicker is that when we compare where the danger line was in the 2001 report and where it is now, it’s come down significantly. This is mainly because we’ve got more evidence of bad stuff that’s happened already, better info on what the future could be like, better understanding of how vulnerable we really are (particularly specific populations like poor and elderly and specific regions like Africa), and better understanding of what it takes to cross those tipping points.
This is our best estimate of what “dangerous” means – at the most 2ºC, maybe lower, which is what we’re committed to already with the warming that’s in the pipeline. Not a good thing.
Okay, so now to #2: What do we have to do to avoid it?
Now we have to translate this 2ºC limit to a CO2 level – or better yet, a CO2-equivalent level.
CO2-eq means taking all the other greenhouse gases (methane, NO2, ozone, CFCs - but NOT water vapor) and adding them into the equation, as well as including the negative effect of aerosols that block sunlight and cause cooling.
If we go with the Momma Bear climate sensitivity, this means about 450 ppm CO2-eq (or 350-400 ppm CO2). This gives us a 50% of staying under 2ºC. And because we’re basically there already and aren’t anywhere near the point of starting to decrease our emissions, we gotta go with the second best option:
Get to peak CO2 emissions ASAP and then make up for being late in getting there by cutting our emissions even faster – one study says lowering them by 5% per year until we get down to 60-80% reduction, at which point the Earth can absorb the rest. This allows us to still limit the total amount of CO2 up there and hopefully stay in the safe zone. (Remember that even as we on the down-side of the emissions curve, after we’ve peaked, we’re still increasing the total amount of CO2 up there.)
The IPCC says that to stay under 450 ppm, developed nations should aim for 35-40% reductions in emissions below 1990 levels by 2020. This is a lot more than what’s being put out on the table right now as possible targets. That means we’ve got a lot of work to do.