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Climate change

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Climate change

Postby kurt_w » Sun Jan 13, 2013 11:13 am

In another thread, John Henry asks some questions about climate change. It's probably pointless trying to have a rational discussion with someone who's prone to lunatic conspiracy theories, but since others might be interested, I thought I'd go ahead and respond. But since the other thread is filed under "National Politics and Government", and I'm not interested in the politics of climate change, I took the liberty of moving it over here.

johnfajardohenry wrote:1) How much global warming has there been since 1900 or so to present? (Degrees C or F)


Since 1900, the global mean land+ocean temperature index has risen at a linear rate of +0.74C/century (95% confidence interval +0.67 to +0.81C/century). Because of the heat capacity of the oceans, the land is warming significantly faster than this, while the ocean is warming more slowly.

However, it's not really appropriate to calculate a linear rate over this period, because the temperature trend was clearly nonlinear: The temperature rose gradually through the 1920s and 1930s, then flattened out until around 1975, when it began rising more rapidly.

Image

The warming in the 1930s appears to be in part caused by volcanic forcing (an unusual absence of large volcanic eruptions that would cool the planet by sporadically injecting plumes of sulfates into the stratosphere).

During the post-WWII era, warming caused by greenhouse gases was initially countered by cooling from anthropogenic aerosols. After the developed countries began reducing air pollution in the 1970s, greenhouse warming became the dominant trend, though it is still partially masked by aerosols, particularly from rapidly industrializing countries in Asia.

johnfajardohenry wrote:2) How much global warming has there been between 1998 and present (C or F)


That's not an appropriate interval over which to estimate global warming. First, it's too short -- the interannual variability from ENSO etc. will mask any long-term trends. Second, you're cherry-picking a starting year that coincides with one of the largest El Ninos of the past century.

Thus, if you calculate a trend from the El Nino year of 1998 to the present, it is +0.73C/century (CI -0.38 to +1.83C/century). But if you start one year later, during the La Nina year 1999, the trend is +1.1C/century (CI 0 to +2.27C/century).

Note the big impact of moving your starting point one year. Note also the excessively wide confidence interval. Both of these are indications that a 14-year period is too short to be estimating long-term rates of warming.

Instead, it makes more sense to examine the rate since the 1970s, when the warming trend from CO2 first exceeds the cooling trend from aerosols. Over the period from 1975 to the present, the trend is +1.66C/century (95% CI of +1.37 to +1.96C/century).

I think that's the most realistic estimate of the late 20th century/early 21st century warming rate, for the globe as a whole. Again, land is warming faster, while the oceans are warming more slowly due to the high heat capacity of water. Most people expect the rate of warming to increase during the 21st century due to the combination of population growth plus industrialization in the developing world.

johnfajardohenry wrote:3) How accurate/precise are the measurements? (+/-) how many degrees C or F)


See confidence intervals above.

johnfajardohenry wrote:4) Have the climate scientists, in the past 20-30 years, been right about any of their predictions? If so which one(s)? For example, about the polar ice cap disappearing in 2012.


Some predictions have been remarkably accurate. The first modern prediction of global warming was in a 1975 Science paper by Wallace Broecker, a geochemist at Columbia University. ("Climatic Change: Are We on the Brink of a Pronounced Global Warming?"). Broecker's predictions are discussed in some detail here and here. Broecker's model omitted several factors that turned out to be important, but those included both warming and cooling factors, so they tended to cancel out and his result is quite close to the actual warming trend:

Image

Likewise, NASA's James Hansen published a paper in 1981 predicting climate change ("Climate Impact of Increasing Atmospheric Carbon Dioxide", discussed here and here). His model's predictions have also turned out quite well, though they under-estimated the observed warming (so far) by about 30%:

Image

Note that both of those predictions were made at what turned out to be the end of a 30-year period of basically flat temperatures. It took some guts to ignore that and say "Over the next 30 years, the world is going to start to warm rapidly, due to CO2 emissions." Broecker and Hansen made some bold predictions that turned out to be right.

In other respects, climate and earth system scientists have tended to underestimate the effects of climate change. For example, sea level has been rising at a rate near the upper end of the range of predictions. Note that the actual observations (red, blue lines) follow the top edge of the gray band of predictions:

Image

Another example is Arctic sea ice. Models predicting sea ice decline have improved a lot, but they continue to underestimate the rate at which sea ice is disappearing:

Image

In that figure, the dotted lines in the background are outputs from various models; the solid black line is the mean of model predictions, and the solid red line is what actually happened.
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Re: Climate change

Postby kurt_w » Sun Jan 13, 2013 11:25 am

Responding to another comment from John Henry:

johnfajardohenry wrote:
Madsci wrote:If we get another summer like the last one, I worry about the reliability of our agricultural system.


It is "the 15th driest year on record". 15th. (According to the link from the NPR report)

That means that there have been 14 worse ones and we made it through OK.

Also, all (most?) of the other 14 occurred in the 1930s and 1950s before we had global warming. So why do you think this one has anything to do with global warming?

I get the impression that you think a single dry year is a trend. Could you explain why, please?

John Henry


Last summer was the 15th lowest precipitation on record in the US, yes. But it was also the #1 warmest year, and drought results from the interaction of high heat + low precipitation.

I haven't been able to find good nationwide historical drought index data for the US going back to the 1930s or earlier. However, there are data broken down for finer-scale areas here. By averaging them, you can get a rough approximation of the history of the US as a whole, though it's not perfect.

By its drought z index, 2012 was the third most extreme drought year on record in the US. It really was an extraordinary combination of high heat plus low precipitation.

However, in general, the post-1970 period for the US as a whole has been one of reduced drought. In the regional data set I mentioned above, of the 20 most widespread and severe drought years, only two occurred after 1965 (1988, and 2012). Most of the rest were in the 1930s and 1950s.
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Re: Climate change

Postby kurt_w » Sun Jan 13, 2013 11:54 am

kurt_w wrote:Instead, it makes more sense to examine the rate since the 1970s, when the warming trend from CO2 first exceeds the cooling trend from aerosols. Over the period from 1975 to the present, the trend is +1.66C/century (95% CI of +1.37 to +1.96C/century).

I think that's the most realistic estimate of the late 20th century/early 21st century warming rate, for the globe as a whole.


So what, exactly, does that mean? We've already had about 0.8 C of warming. Extrapolating from the past 40 years would suggest a total of around 2.3C by 2100, or perhaps 3C or more if the rate accelerates (as one would expect, given the development of China, India, etc.)

Is that a lot? Global mean land+ocean temperature is not exactly an intuitive number. So let's compare it to past changes in the climate:

* At the last glacial maximum, when most of Wisconsin was covered by a few thousand meters of ice, global mean temperature was probably 4C to 7C colder (one recent paper says 2.6C colder, but that's an outlier and I don't find it credible).

* During the period since humans developed agriculture, most multi-decadal changes in climate have been relatively small. For example, the so-called "Little Ice Age" in the 18th and 19th centuries was characterized by a drop in global mean temperature of about 0.5C.

So anticipated warming in the 20th/21st centuries would be approximately five to six times as large as the cooling during the "Little Ice Age", and approximately one-half to one-third as large as the change from the last glacial maximum to the present.
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Re: Climate change

Postby kurt_w » Sun Jan 13, 2013 12:35 pm

Next up: Why climate scientists think the earth will continue warming.

Most of the first post in this thread was about how much warming has occurred in the past. But scientists don't base their expectation of future warming just on extrapolating from the past.

The real reason for anticipating future global warming is just a straightforward line of reasoning from first principles:

(1) Humans are burning fossil fuels.
(2) When burned, fossil fuels produce CO2.
(3) This increases the concentration of CO2 in the atmosphere.


Sure enough, the CO2 concentration in the atmosphere is increasing as expected:

Image

This change is a pretty radical departure from past history:

Image

Knowing about how much fossil carbon is being burned each year, it's possible to calculate that about half of the CO2 we're emitting gets stored in the oceans, biosphere, and soils, while the other half remains in the atmosphere.

Next, we know what effects CO2 will have in the atmosphere:

(4) CO2 molecules absorb and emit infrared radiation.
(5) Increasing the CO2 concentration in the atmosphere will raise the altitude at which the atmosphere emits radiation to space.
(6) Raising that altitude will increase the surface temperature (via the lapse rate).


So far, none of these six facts is particularly controversial. Some fairly straightforward calculations yield a first estimate of the climate sensitivity (the amount of warming that would result from doubling CO2) of 1C in the absence of feedbacks.

There are both positive and negative feedbacks in the climate system. Negative feedbacks tend to stabilize the climate, while positive feedbacks tend to amplify any changes. Some feedbacks are well understood, while others are less certain.

For example, water vapor is a known positive feedback. As the planet warms (or cools) the atmosphere can hold more (or less) water vapor. Since water vapor itself is also a greenhouse gas, this increase (or decrease) in water vapor adds an additional amount of warming (or cooling) on top of the original perturbation.

The only way that doubling CO2 could produce less than 1C of warming would be if the sum of all feedbacks was negative. This would tend to stabilize the climate. But we know that in the past, climate has varied quite a bit (ice age cycles, the Little Ice Age, the Medieval Warm Period, etc.) This implies that the climate system can't be dominated by negative feedbacks.

I don't think there's anything in this chain of reasoning that can seriously be questioned. Therefore, doubling CO2 must warm the planet by more than 1C. The only real areas for debate are:

(7) How much more than 1C would result from doubling CO2? [Best estimates appear to be 2.5 to 3C per doubling]
(8) How much CO2 will human activities produce during the next century? [This is difficult to predict. People are working on it]
(9) What will be the economic and environmental impacts from the resulting warming? [This is also difficult. People are working on it]
(10) How should human societies respond to these anticipated impacts? [This is a question of values, not scientific facts]

Again, (1) through (6) are pretty much not up for dispute. The scientific uncertainty is all in (7) through (9) and the policy uncertainty is in (8) through (10).
Last edited by kurt_w on Wed Oct 23, 2013 6:41 am, edited 1 time in total.
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Re: Climate change

Postby Galoot » Sun Jan 13, 2013 1:28 pm

Excellent work, Kurt, thanks for putting in the time to post this.
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Re: Climate change

Postby snoqueen » Sun Jan 13, 2013 1:53 pm

And in reply to a comment from another topic: lack of responses doesn't mean we didn't get anything out of this.
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Re: Climate change

Postby Bland » Sun Jan 13, 2013 3:17 pm

snoqueen wrote:And in reply to a comment from another topic: lack of responses doesn't mean we didn't get anything out of this.
And by "we" sno means "everyone but John Henry"
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Re: Climate change

Postby snoqueen » Sun Jan 13, 2013 5:08 pm

Is there anyone else in the readership who doesn't already accept the reality of climate change?
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Re: Climate change

Postby Madsci » Tue Jan 29, 2013 9:25 pm

A belated thanks to Kurt W for taking on the troll.

My favorite story as a child was Three Billy Goats Gruff. I let my brother take on the trolls on this forum. So Kurt, you are now an honorary brother.
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Re: Climate change

Postby gargantua » Tue Jan 29, 2013 10:30 pm

Thanks Kurt.

Anyone know if the oceans and biosphere can continue to store 50% of increased CO2? If we reach a limit of the ocean's carrying capacity, atmospheric CO2 would increase even more rapidly, right?

Sorry if that's a stupid question....saw it in an old sci-fi book I read. It did not go well for us.
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Re: Climate change

Postby pjbogart » Tue Jan 29, 2013 11:31 pm

Madsci wrote:A belated thanks to Kurt W for taking on the troll.

My favorite story as a child was Three Billy Goats Gruff. I let my brother take on the trolls on this forum. So Kurt, you are now an honorary brother.


I was thinking more along the lines of "mess with the bull, get the horns."

Regardless of how Jason finishes up his list, the top spot will be wrong. It was Kurt. It's always been Kurt.
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Re: Climate change

Postby Mad Howler » Wed Jan 30, 2013 12:38 am

Kurt_w,
Fuck! I was sitting around last night "enjoying" the January thunderstorm and down pour and wondering why nobody here was talking about it, "it" being roundly addressed well before. What a dick I am, I totally blew it and missed these posts. Excellent work laying bare the reality of what we currently understand of this situation.

I do have a couple of questions. At what point can we predict that the dissolution of CO2 in the oceans of the planet will seize to have a role in negative feedback and turn to positive? Also, what is the current thinking regarding these changes and the potential release of less soluble but more potent gases such as CH4 or hydrogen sulfide?

MH
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Re: Climate change

Postby green union terrace chair » Wed Jan 30, 2013 1:35 am

Kurt, I especially appreciate your attention to the importance of sample size. Two dumb comments I equally hate to hear:
  • When it's an unusually warm winter day:
    -- "OMG, GLOBAL WARMING!!! :shock: "
  • When it's a particularly cold winter day:
    -- "LOL, GLOBAL WARMING :lol: "
A single data point does not an argument make.
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Re: Climate change

Postby Mad Howler » Wed Jan 30, 2013 2:04 am

green union terrace chair wrote:Kurt, I especially appreciate your attention to the importance of sample size. Two dumb comments I equally hate to hear:
  • When it's an unusually warm winter day:
    -- "OMG, GLOBAL WARMING!!! :shock: "
  • When it's a particularly cold winter day:
    -- "LOL, GLOBAL WARMING :lol: "
A single data point does not an argument make.


Kw's OP was January 13th so I do not understand your point. Although, I think I have an idea where this thread might be heading.
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Re: Climate change

Postby Sandi » Wed Jan 30, 2013 3:15 am

snoqueen wrote:Is there anyone else in the readership who doesn't already accept the reality of climate change?


The question isn't one of climate change. It never has been, and I don't think there is anyone is the world would argue any different. The question is whether we as humans are responsible for any of it, and if so, how much.

If the earth has warmed in the last century, is that because of industry, or because of normal climate cycles? Models are fine, but they can only put so much of the overwhelming data that affects climate into them. Sun activity is one piece of data left out of most models.

Another problem is the politicizing of the grant system, and that goes for all fields of science, and it is even worse in medical science. If you want a grant, your research needs to find this or that political view. Bring in the wrong results, and you can forget about future grants.

Climate change is real, has been from the dawn of life on earth and before. Do I think man is responsible for recent climate change? Possibly. But we don't have enough information to show one way or the other.

Kurt has done some nice work that shows climate change, but it doesn't show the cause. It simply can't.
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