Archive for December, 2018

If you pay attention to the media reporting on “climate change” (note 1) you will often read/hear something like this:

Under a business as usual scenario..

And then some very worrying future outcomes. Less misleading, but still very misleading, you might read:

Under a high emissions scenario..

Every time I have checked the papers that are behind the press release faithfully reproduced by the stenographers in the media, they are referring to a model simulation using a scenario of CO2 emissions known as RCP8.5.

This scenario – explained below – is a fantastic scenario, and was not created because it was expected to happen.

So – calling it “business as usual”, charitably speaking, is from climate scientists who know nothing about history, demography, current & past trends. And uncharitably, is from climate scientists who are activists, pressing a cause, knowing that stenographers don’t do research or ask difficult questions.

I have been reading climate science for a long time – textbooks, papers, IPCC reports – and for sure when I read “under a business as usual scenario..” I always thought that climate scientists meant – “if we continue doing what we are doing, and don’t immediately reduce CO2 emissions”.

Then I read the papers on the scenarios.

Let me explain. It’s worth spending a few minutes of your time to understand this important subject..

Pre-industrial levels of CO2 were about 280ppm. Currently we are at just over 400ppm. Half a century ago climate scientists used rudimentary climate models and tried doubling CO2 to find out what the new climate equilibrium would be. Some time later people tried 3x and 4x the amount of CO2. It’s a good test. Do the simulated effects of CO2 on climate keep on increasing once we get past 2x CO2? Do the effects flat-line? Skyrocket?

Very worthwhile simulations.

Today we have lots of climate models. There are about 20 modeling centers around the world, each producing results that often vary significantly from other groups (more on that in future articles). How can we compare the results for 2100 from these different models? We need to know how much CO2 (and methane) will be emitted by human activity. We need to know land use and agricultural changes. Of course, no one knows what they will be, but for the purposes of comparison different modeling groups need to work from identical conditions (note 2).

So a bunch of scenarios were created, too many probably. It takes lots of computing power to run a simulation for 100 years. For IPCC AR5 in 2013 these were slimmed down to four Representative Concentration Pathways, or RCPs. One of these is RCP8.5.

The paper writers didn’t come up with RCP8.5 because they felt this was a likely scenario. They were told to come up with RCP8.5:

By design, the RCPs, as a set, cover the range of radiative forcing levels examined in the open literature and contain relevant information for climate model runs..

..The four RCPs together span the range of year 2100 radiative forcing values found in the open literature, i.e. from 2.6 to 8.5 W/m². The RCPs are the product of an innovative collaboration between integrated assessment modelers, climate modelers, terrestrial ecosystem modelers and emission inventory experts. The resulting product forms a comprehensive data set with high spatial and sectoral resolutions for the period extending to 2100..

..The RCPs are named according to radiative forcing target level for 2100. The radiative forcing estimates are based on the forcing of greenhouse gases and other forcing agents. The four selected RCPs were considered to be representative of the literature, and included one mitigation scenario leading to a very low forcing level (RCP2.6), two medium stabilization scenarios (RCP4.5/RCP6) and one very high baseline emission scenarios (RCP8.5).

From IPCC AR5, chapter 2, p.167 we can see that the change in CO2 per year (the bottom graph) is about 2ppm. The legend means, in English rather than maths, the increase in CO2 concentration in ppm per year:

So a naive expectation based on current increases would be around 570 ppm in 2100 (410ppm + 80 years x 2ppm)

This is pretty close to the scenario RCP6, and a very long way from RCP8.5.

To get to RCP8.5 requires almost 1000ppm of CO2 (plus large increases in methane concentrations). This requires about 7ppm per year increase in CO2 starting soon. It’s a “fantastic” scenario that is extremely unlikely to happen, and if by some strange set of circumstances it was, the world could stop it simply by ensuring that sub-Saharan Africa had access to cheap natural gas, rather than coal (see note 3).

If climate scientists and media outlets wrote “under a very unlikely emissions scenario that we can’t see happening we get a few outlier models that predict..” it wouldn’t make good headlines.

It wouldn’t make good climate advocacy.

When you see a story about possible futures, check what scenario is being used. If it’s RCP8.5 (“business as usual” or “a high emissions scenario”) then you can just ignore it – or be concerned and start petitioning your government to encourage more natural gas production.

– Update Jan 1, 2019 (Dec 31st, 2018 in some parts of the world) -just added Opinions and Perspectives – 3.5 – Follow up to “How much CO2 will there be?” due to comments

Further Reading

Impacts – II – GHG Emissions Projections: SRES and RCP


Note 1: I put “climate change” in quotes to distinguish it from climate change that happened up until 1900 or thereabouts. I’m trying to keep this series non-technical, and also assume that readers haven’t read/remembered previous articles in the series. See Opinions and Perspectives – 2 – There is More than One Proposition in Climate Science

Note 2: A significant part of climate modeling is assessing results and trying to figure out why, say, the GISS model differs substantially from the MPI model. To do that we need to be sure that the model results are based on the same conditions.

Note 3: Natural gas produces about 1/2 the CO2 of coal, per unit of energy produced. If you read the paper for RCP8.5 you will see it depends upon a very high sub-Saharan African population burning huge amounts of coal. No demographic transition. No technological progress. A Victorian technology.

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Continuing from Opinions and Perspectives – 1 – The Consensus a friend said to me a little while back, “Oh, you don’t believe in climate change do you?”

Ye gods, where to start?

At some exhibition which included a questionnaire that visitors were encouraged to take, one of the later questions was “Do you believe in climate change?”. My uncle remarked, “A question that reveals more about the questioner than about the respondents”. I wish I had his gift.

Let me outline some propositions required for basic climate literacy. That is, whether you agree or not with these propositions, you should know that they are distinct, and important:

1. Before “climate change” there was lots of climate change. That is, before humans began emitting large quantities of CO2 (and other GHGs) by burning fossil fuels the climate experienced large changes on time scales ranging from decades to centuries to millennia and longer.

2. Burning fossil fuels like coal and natural gas adds CO2 to the atmosphere.

3.  More CO2, methane and a few other inappropriately-named “greenhouse” gases in the atmosphere increase the surface temperature of the earth.

Items 2 and 3 above can be summarised with the term “anthropogenic global warming” or AGW.

4. Just because there was lots of climate change before AGW doesn’t mean that humans can’t alter the climate.

5. AGW will lead to catastrophe for our planet (perhaps we can call this CAGW).

Each one of these propositions is distinct. And proposition 5 could be broken down into a number of different propositions (which we will look at).

For example, many people “believe in climate change” while refuting even AGW. Their argument is sometimes, “The climate was changing long before we started burning fossil fuels, that’s why I don’t believe in AGW”.

I don’t share that point of view. But wrapping causes around catchy phrases can, of course, backfire.

It is possible to believe in proposition 2 and not proposition 3. It is possible to believe in AGW (2 & 3) and not proposition 5.

Most people, after at least a decade and a half of the media blaring at them (from whatever ideological position), don’t realize that these propositions are not all: “Do you believe in climate change?”

It’s almost as though the media is completely counter-productive for grasping complex issues.

Note to commenters – if you want to question the “greenhouse” effect post your comment in one of the many articles about that, e.g. The “Greenhouse” Effect Explained in Simple Terms. Comments placed here on the science basics will just be deleted.

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When I started this blog I said:

Opinions are often interesting and sometimes entertaining. But what do we learn from opinions? It’s more useful to understand the science behind the subject.

Of late I’ve been caught up with work, other intellectual interests and (luckily) some fun stuff and haven’t spent any time on climate. So I feel it’s time to put forward a few opinions on climate.

The often-cited consensus on climate is:

a) we add CO2 and other GHGs to the atmosphere by burning fossil fuels (and other human activity)

b) these increase the inappropriately-named “greenhouse effect”

c) this increases the surface temperature over some time period

This scientific consensus is rock solid, like gravity or momentum. It’s not particularly intuitive, but tough luck, physics is often like that. By itself, the consensus doesn’t tell you a lot. It just says that if we keep burning fossil fuels then the earth’s surface temperature will increase.

This scientific consensus doesn’t say that urgent action is needed on climate, or that without urgent action society is doomed, or that rapid adoption of renewable energy towards 100% of current energy consumption is a net cost benefit.

These are different propositions.

Note to commenters – if you want to question the “greenhouse” effect post your comment in one of the many articles about that, e.g. The “Greenhouse” Effect Explained in Simple Terms. Comments placed here on the science basics will just be deleted.


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