This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.
The science behind global warming is, well, science, and it can get pretty technical. By how many degrees has the globe already warmed? How much more can it warm before we’re in trouble? How much carbon dioxide is in the air now, and how much more can we afford to emit before risking climate catastrophe? Which are the most important greenhouse gases? And what do all those funny abbreviations mean?
Below you will find a handy crib sheet that gives you all these numbers and more.
Temperatures
Two factors frequently confuse discussions of temperature:
- What is the baseline temperature we’re comparing to, preindustrial or current?
- Is the temperature scale Fahrenheit or Celsius?
And then there’s the issue of "warming in the pipeline". Temperature may have warmed by only a certain amount today, but some additional warming is certain because warming lags behind greenhouse gas emissions. First greenhouse gas concentrations increase, and then some time later warming occurs. The amount of future warming that is certain due to existing greenhouse gas concentrations is called "warming in the pipeline".
Discussions of global temperature often center around "tipping points" – the points after which qualitative climate change become inevitable. The most commonly cited of these is the melting of the Greenland Ice Sheet, which would lead to a 20-foot rise in sea levels, so this is what is shown in the table below. (For more, see last week’s post on "tipping elements".)
| Warming | Temperature above Pre-industrial (Celsius) | Temperature above Pre-industrial (Fahrenheit) |
|---|---|---|
| Since pre-industrial | 0.7 | 1.3 |
| In the pipeline | 0.6 | 1.0 |
| Total commitment | 1.3 | 2.3 |
| Likely tipping point for Greenland Ice Sheet | 2.0 | 3.6 |
Greenhouse Gas Notation
For the rest, let’s start with vocabulary. When you hit an abbreviation you don’t understand, it’s hard to follow the rest of the discussion.
| ppm | parts per million – to describe atmospheric concentrations of greenhouse gases. |
|---|---|
| MMT or Mt | million metric tons or megatons – to describe the amount of greenhouse gas emissions spewed into the atmosphere. |
| Gt | billion metric tons or gigatons – equivalent to 1000 Mt |
| CO2 | carbon dioxide, an important greenhouse gas. |
| CO2e | carbon dioxide equivalents – to describe all greenhouse gases in terms of the warming potential of carbon dioxide (some greenhouse gases cause more warming, ton for ton, though there are less of them). |
Greenhouse Gas Warming Potentials
There are dozens of human-produced greenhouse gases, but the three that account for the most warming are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Although CO2 causes the most warming, that’s because it’s the most prevalent, not the most potent. Methane and nitrous oxide have much higher warming potentials.
Greenhouse Gas Percents by Volume
| Greenhouse Gas (GHG) | Percent of Long-Lived GHGs | Warming Potential of Gas vs. CO2 | Percent Warming Caused by Gas |
|---|---|---|---|
| carbon dioxide | 99.4% | 1 | 62.9% |
| methane | 0.5% | 25 | 18.2% |
| nitrous oxide | 0.08% | 298 | 6.1% |
Greenhouse Gas Concentrations
The concentration of greenhouse gases in the atmosphere drives global warming. Here’s a table showing where we’ve been, where we are, and where we’re headed if we continue with business-as-usual.
| GHG Concentration (CO2 or CO2e) | Year | Notes |
|---|---|---|
| 280 ppm CO2 | 1750 (Preindustrial) | |
| 383 ppm CO2 or CO2e | 2008 | It’s approximately the same in CO2 or CO2e because of the cooling effects of "aerosols" (for example, the sulfate particles Bill discussed in his post on geoengineering). |
| 450 ppm CO2e | 2040, in one business-as-usual scenario* | Gives a 50 percent chance of exceeding +2°C, a commonly cited tipping point for the Greenland Ice Sheet (see above). |
| 1000 ppm CO2 | 2100, in one business-as-usual scenario.* |
*See figures 10.20 and 10.21 in IPCC’s 4th Assessment Report. Note that these graphs show CO2 only. Business-as-usual scenarios project that CO2e could increase even more quickly than CO2 alone.
Greenhouse Gas Emissions
These statistics are from 2005, which is the most recent data available.
Global Emissions
| CO2 emissions from fossil fuels | 28.2 Gt | CO2 |
| Emissions of non-CO2 greenhouse gases | 10.2 Gt | CO2e |
| Emissions from deforestation (17% of total) | 7.9 Gt | CO2 |
| Total greenhouse gas emissions | 46.3 Gt | CO2e |
U.S. Emissions
- Total emissions: 7.2 Gt CO2e
- Increase since 1990: 16 percent
Emissions Reductions
What emissions reductions are necessary to keep greenhouse gas concentrations below 450 ppm? This table summarizes the cuts we need to make in the next few decades, though of course our efforts will have to continue beyond 2050.
Emissions Cuts Needed to Stop Warming
| Year | Developed Countries | Global Emissions |
|---|---|---|
| 2020 | 20 percent below 2005 | Peak and begin to decline |
| 2050 | 80 percent below 2005 | 50 percent below 2005 |
Are there any other basic statistics you’re wondering about? Post a comment – I’d be glad to explain.
8 Comments
how do you explain the fact that in ice core samples from both GReenland and Antarctica over the last 500,000 years or so, warming occured FIRST and then CO2 levels rose several hundred of years later. therefore one can surmise that CO2 is an EFFECT of warming and not a cause of it. also, there have been times in our past when it was warmer than it is today and CO2 levels were lower. there have also been times when it was cooler than it is today and CO2 levels were higher. so i am not sure what you are trying to explain here. I would welcome your comments on my comments above.
Nice cribsheets, although you might want to say something about the lifetimes of the various GHGs in the atmosphere.
fred1: your question is #8 [co2lag] in the list of standard arguments in:
http://www.skepticalscience.com/argument.php
>you might want to say something about the lifetimes of the various GHGs in the atmosphere.
That’s a good addition! I’ll ask Lisa to put a post together on that.
And thanks for supplying fred1 with a link – you saved me the work. :)
Sorry for the delay in responding. Johnmashey, i would recommend you read thoroughly the blog comments in the #8 you reference. the comments actually validate my statement. i am not sure what you are trying to state. true some additional CO2 feedback may be possible to slightly amplify warming induced by other factors. however, the majority of the comments by other obviously rather well informed individuals in #8 show that there is no real consensus.
i would also, again, ask the question, if increased CO2 levels are a main driver of global warming, then how can it be that 1000 years in the Medieval Warming period, that temps were considerably higher than they are today, yet CO2 levels were lower than today. BTW, if you are inclined to quote the latest IPCC report as not showing the medieval warming period, i would call your attention to the IPCC report historical temp data chart in 1995, which clearly showed the medieval warming as well as the little Ice Age.
also, since we are talking about skeptical science (thanks for pointing that website out to me BTW), lets face it, there is no way that the urban heat island effect is not a factor with respect to higher temp readings at stations. the latest IPCC report disclaims this saying it is miniscule. all you have to is to watch the evening weather on the local news in any major metro area. what is said, lows around XXX in the city, lows around XXX-5-10 in the suburbs. obviously the urban heat island effect is true and significant. also, here is a good one for you, check out surfacestations.org. started by a weather guy who in CA. no political agenda whatsoever, was just curious what the condition was of the actual temp station where he got his readings from for his newscast. turns out, his organization has evaluated almost half of the “official temp” stations in the U.S. and found that of those almost half are out of compliance in terms of distance from heat sources (asphalt, AC vents, others, etc.), not painted or ventilated properly, etc. what this really means is that 30% of the temp reading stations in the U.S. are likely not accurate in terms of temperatures. so we really can’t even trust the most basic data that allegedly implies warming. check out the site.
Fred1:
1) As you’ve probably noted, I suggested the Lorius paper to John Cross, and I’ve read the comments. The reason I often reference John’s website is:
a) He gives a nice concise description, reasonably accessible to a non-expert, with appropriate caveats regarding which parts of the problem are really well-understood, and which aren’t.
b) Most of his references are to peer-reviewed primary research in top journals like Nature and Science, or secondary (but still very credible sources) like IPCC reports, or US NAS (National Academy of Sciences) / National Research Council.
I have the IPCC TAR & AR4, and some NRC reports. I don’t get Nature, but I’m a AAAS member, so I read Science every week, and when there are arguments amongst scientists there about these topics, I follow them as they happen, or I ask relevant scientists directly.
2) Compare the credibility of:
a) Some comments by unidentified blog posters.
with:
b) Strong consensus in the peer-reviewed literature about this topic, which I find in the sources above, plus talking to members of the NAS, IPCC authors, and other scientists. Physics Nobel winner Burton Richter, who lives not far away. He gave a good global warming talk in our little town a few years ago. This exact topic came up, and he gave the same explanation as Lorius & Hansen.
I’ve heard similar talks from several IPCC authors, NAS members, and senior researchers at places like Stanford, MIT, and Imperial College (“MIT of the UK”). If they didn’t mention it themselves, somebody always asks, and the answers were the same.
Gore’s AIT talk was (unsurprisingly) similar to all these, albeit pitched for a broader audience. I wish he’d taken the extra 2 minutes to explain the CO2-amplifier effect in ice-age terminations, since I think that’s where the questions come from.
Anyway, if you *want* to believe there’s no *scientific* consensus on this because some blog posters argue about it, you can, but I wouldn’t. If a bunch of topnotch doctors tell me “you need an operation, or you can wait to have another event (doctor-speak for a heart attack that will probably kill or cripple you”, I might research the problem a bit, but I’d be very likely to take their advice. I wouldn’t post a question on a blog, and seeing arguments from unidentified posters, conclude there there was no consensus. [This isn’t theoretical.]
Anyway, by scientific standards, the posters on that thread were not necessarily well-informed. At least some appeared to be obfuscatory, but in any case, if someone can provide a credible disproof of this [again, it’s been around since 1990 or before], all they have to do is disprove it, write it up for Science or Nature, get it published, and they will be *famous*. Any climate scientist I’ve ever talked to [a lot] would love to do that … except the physics is straightforward enough [Greenhouse Effect is real, CO2 solubility in water decreases with increasing temperature] that nobody serious would waste time doing it.
(more later)
Fred1: next part, of several: MWP
1) SkepticalScience’s #2 is a reasonable, if short treatment.
But I’ll give you a couple more.
2) Every climate scientists I’ve talked to about this [I used to sell supercomputers to places like NCAR, GFDL, NASA, etc] says that GHGs in general and CO2 in particular are important drivers, but nobody thought those were the only ones, or expected measured surface temperatures to correlate perfectly with CO2 levels. Among other things:
– external influences, i.e., solar irradiance and Milankovitch cycles
– internal oscillations in the ocean/atmospheric systems, like El Nino/la Nina
– volcanoes
– major state changes, likely in ocean flows, like the Younger Dryas
– various kinds of feedbacks, like the CO2-amplification, the ice-albedo feedback, which of course amplify in both directions, which creates the spikieness in Earth’s temperatures.
Anyway, all of this creates a “noisy time series”, and unless they have training & experience with such, most people’s intuition is simply wrong. http://tamino.wordpress.com/ has especially good articles about the issues in analyzing such series. The CO2 series is *not* very noisy; the temperature series is.
2) I strongly recommend William Ruddiman’s “Plows, Plagues, and Petroleum”, and I put a review on Amazon, so won’t repeat that.
http://www.amazon.com/Plows-Plagues-Petroleum-Control-Climate/dp/0691133980/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1204132075&sr=8-1
Subject to the usual noise issues, absent human effects, the MWP *should* have been (slightly) warmer than now: See Ruddiman page 108. From Milankovitch Cycles, the relevant solar insolation was higher then. In fact, Ruddiman makes a pretty good case that without humans putting CO2 and CH4 into the atmosphere for thousands of years, we’d have already begun the long, slow (thousands or tens of thousands of years) descent back to ice-covered Canada. See chart on page 145. People argue about the details, but I’ve read the detailed, peer-reviewed papers, and they look pretty good.
3) In any case, going forward, it DOES NOT MATTER whether the MWP was global or more limited in geography, and it DOES NOT MATTER whether it was warmer then or not. [Mixed eveidence: I think it was more limited, and very likely warmer, but I don’t care.] People want to say:
– well if it was warmer, somewhere, then, then all this must be natural, and the next swing will take us back, and All Will Be Well.
– Unfortunately, anyone with a good background in physics & statistics doesn’t have the luxury of believing that.
– In particular, the planet holds 10X more people than it did 1000 years ago, and a vast amount of infrastructure built at or near sealevel.
As to why it doesn’t matter, I’ll give several levels of argument.
3a) What effect does knowing the temperature in 1000AD have on the temperature in 2008AD?
A: none.
Suppose we could go back to 10,000BC, and cover the Earth and ocean with thermometers, and fill the sky with climate satellites, so that we had great records. Climate scientists would kill for that data. Would knowing it make the *slightest* difference in today’s temperature?
A: no
Would it make any difference to the actual temperature in 2050 or 2100, given any particular emissions scenario?
A: no.
Would it help us reduce the uncertainty of *predictions* of the average temperatures of 2050 or 2100?
A: probably yes, we’d get tighter bounds.
3b) Thermostat analogy.
The temperature outside your 4-person house jiggles constantly, on a seasonal cycle, on night-and-day, and with heat waves and cold spells, which quite often aren’t synchronized with everyone else in the world.
A few years ago, the climate provided a long warm spell, naturally, and it reached X degrees, where X is about your limit of comfort before you’d turn on the air conditioning.
Now, the temperature inside the house is Y, around X. It’s going up, fast (although with usual natural noisy time-series jiggles) because you have a funny thermostat that is ratcheting up over time, and the furnace keeps trying to catch up and get back into equilibrium.
The thermostat’s case appears to be locked, and its maximum temperature limit is rather unpleasant. Even if you get into the case and stop the ratchet, the furnace is lagged and will keep running. The air conditioner is broken, and you now have +36 relatives living with you.
How much does it matter whether Y is > X right now?
A: zero. The house temperature is going UP regardless of what the natural heatwave did years ago.
3c) The physics version, quickly:
First Law of Thermodynamics [Conservation of Energy]
Second Law of Thermodynamics
The Greenhouse Effect
Oceans get warmed by sun and by warmer air, but the “ocean” doesn’t have one universal temperature, and it has cold currents and warm currents and osciallations [like El Nino] that can suddenly warm the whole planet, at least as measured on the surface?
Do you believe those?
Then: here’s a simple explanation. With jiggles and various lag times, more CO2 => warmer atmosphere, i.e., Greenhouse Effect, although misnomered, is real. The First Law says the heat is somewhere. We have moderately good temperature records on land, less good ones on the sea surface, and far less good in the deeper ocean, and those, only recently. An El Nino year (like 1998) shows what happens when an oscillation causes the ocean to give up more of its heat to the atmosphere.
The Greenhouse Effect, and rising CO2, guarantee that the heat content of (ocean + atmosphere) keep rising (for a long time, but not into Venus-style runaway). However, given the oscillations, the split between ocean and atmosphere jiggle around, and most people only notice the latter, despite the *obvious* effects of El Nino / LA Nina. The total heat content of the Earth did *not* change radically from 1997 through 1999, did it? The Sun was still shining about the same, and orbital mechanics didn’t change drastically. CO2 went up a little. BUT, the (air) temperature spiked from the El Nino.
Right now, we’re at a low point in the sunspot cycle, and we have a La Nina. BUT, there is more total energy in the ocean+atmosphere system than there was in 1998, it’s just that a bigger fraction is in the ocean. It will be interesting to see what happens on the next big El Nino. it takes a long time to warm the deep oceans 9but we’re starting to do it), and of course (First Law again), once you do it, it takes a long time for them to cool off again.
Does that help?
did you check out surfacestations.org?
JM,
oh also you mention that the MWP was likely more warmer than it is today. did the Greenland ice pack suffer an “irreversible meltdown” as mentioned in one of these earlier articles as being a catastrophic event. as far as i know it did not, therefore how are temps that would be 2 degrees F warmer than they are today going to cause a tipping point and irreversibly meltdown of the ice pack if that that never happened during the MWP?