












And he goes on:






























Tactics for Detecting Crap on the Internet


  1. Authorship
    • What do others say about the author?
  2. Sources cited
  3. Ownership of the website
  4. Connectedness
  5. Design & Interactivity
  6. Triangulation
  7. Check your own biases

(Download the full worksheet)


Here’s my abstract for the conference:

Constructive and External Validity for Climate Modeling




For more background, see:




Steve Easterbrook, University of Toronto


























































Weather as an initial value problem


Climate as a boundary value problem


Climate change as a change in boundary conditions








I’ve been trawling through the final draft of the new IPCC assessment report that was released last week, to extract some highlights for a talk I gave yesterday. Here’s what I think are its key messages:

  1. The warming is unequivocal.
  2. Humans caused the majority of it.
  3. The warming is largely irreversible.
  4. Most of the heat is going into the oceans.
  5. Current rates of ocean acidification are unprecedented.

Before I elaborate on these, a little preamble. The IPCC was set up in 1988 as a UN intergovernmental body to provide an overview of the science. Its job is to assess what the peer-reviewed science says, in order to inform policymaking, but it is not tasked with making specific policy recommendations. The IPCC and its workings seem to be widely misunderstood in the media. The dwindling group of people who are still in denial about climate change particularly like to indulge in IPCC-bashing, which seems like a classic case of ‘blame the messenger’. The IPCC itself has a very small staff (no more than a dozen or so people). However, the assessment reports are written and reviewed by a very large team of scientists (several thousands), all of whom volunteer their time to work on the reports. The scientists are are organised into three working groups: WG1 focuses on the physical science basis, WG2 focuses on impacts and climate adaptation, and WG3 focuses on how climate mitigation can be achieved.

Last week, just the WG1 report was released as a final draft, although it was accompanied by bigger media event around the approval of the final wording on the WG1 “Summary for Policymakers”. The final version of the full WG1 report, plus the WG2 and WG3 reports, are not due out until spring next year. That means it’s likely to be subject to minor editing/correcting, and some of the figures might end up re-drawn. Even so, most of the text is unlikely to change, and the major findings can be considered final. Here’s my take on the most important findings, along with a key figure to illustrate each.

(1) The warming is unequivocal


Observed globally averaged combined land and ocean surface temperature anomaly 1850-2012. The top panel shows the annual values; the bottom panel shows decadal means. (Note: Anomalies are relative to the mean of 1961-1990).


Unfortunately, there has been much play in the press around a silly idea that the warming has “paused” in the last decade. If you squint at the last few years of the top graph, you might be able to convince yourself that the temperature has been nearly flat for a few years, but only if you cherry pick your starting date, and use a period that’s too short to count as climate. When you look at it in the context of an entire century and longer, such arguments are clearly just wishful thinking.

The other thing to point out here is that the rate of warming is unprecedented. “With very high confidence, the current rates of CO2, CH4 and N2O rise in atmospheric concentrations and the associated radiative forcing are unprecedented with respect to the highest resolution ice core records of the last 22,000 years”, and there is “medium confidence that the rate of change of the observed greenhouse gas rise is also unprecedented compared with the lower resolution records of the past 800,000 years.” In other words, there is nothing in any of the ice core records that is comparable to what we have done to the atmosphere over the last century. The earth has warmed and cooled in the past due to natural cycles, but never anywhere near as fast as modern climate change.

(2) Humans caused the majority of it

The summary for policymakers says “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century”.

The Earth's energy budget from 1970 to 2011. Cumulative energy flux (in zettaJoules!) into the Earth system from well-mixed and short-lived greenhouse gases, solar forcing, changes in tropospheric aerosol forcing, volcanic forcing and surface albedo, (relative to 1860–1879) are shown by the coloured lines and these are added to give the cumulative energy inflow (black; including black carbon on snow and combined contrails and contrail induced cirrus, not shown separately).


This chart summarizes the impact of different drivers of warming and/or cooling, by showing the total cumulative energy added to the earth system since 1970 from each driver. Note that the chart is in zettajoules (1021J). For comparison, one zettajoule is about the energy that would be released from 200 million bombs of the size of the one dropped on Hiroshima. The world’s total annual global energy consumption is about 0.5ZJ.

Long lived greenhouse gases, such as CO2, contribute the majority of the warming (the purple line). Aerosols, such as particles of industrial pollution, block out sunlight and cause some cooling (the dark blue line), but nowhere near enough to offset the warming from greenhouse gases. Note that aerosols have the largest uncertainty bar; much of the remaining uncertainty about the likely magnitude of future climate warming is due to uncertainty about how much of the warming might be offset by aerosols. The uncertainty on the aerosols curve is, in turn, responsible for most of the uncertainty on the black line, which shows the total effect if you add up all the individual contributions.

The graph also puts into perspective some of other things that people like to blame for climate change, including changes in energy received from the sun (‘solar’), and the impact of volcanoes. Changes in the sun (shown in orange) are tiny compared to greenhouse gases, but do show a very slight warming effect. Volcanoes have a larger (cooling) effect, but it is short-lived. There were two major volcanic eruptions in this period, El Chichón in 1982 and and Pinatubo in 1992. Each can be clearly seen in the graph as an immediate cooling effect, which then tapers off after a a couple of years.

(3) The warming is largely irreversible


(Fig 12.43) Results from 1,000 year simulations from EMICs on the 4 RCPs up to the year 2300, followed by constant composition until 3000.


The conclusions about irreversibility of climate change are greatly strengthened from the previous assessment report, as recent research has explored this in much more detail. The problem is that a significant fraction of our greenhouse gas emissions stay in the atmosphere for thousands of years, so even if we stop emitting them altogether, they hang around, contributing to more warming. In simple terms, whatever peak temperature we reach, we’re stuck at for millennia, unless we can figure out a way to artificially remove massive amounts of CO2 from the atmosphere.

The graph is the result of an experiment that runs (simplified) models for a thousand years into the future. The major climate models are generally too computational expensive to be run for such a long simulation, so these experiments use simpler models, so-called EMICS (Earth system Models of Intermediate Complexity).

The four curves in this figure correspond to four “Representative Concentration Pathways“, which map out four ways in which the composition of the atmosphere is likely to change in the future. These four RCPs were picked to capture four possible futures: two in which there is little to no coordinated action on reducing global emissions (worst case – RCP8.5 and best case – RCP6) and two on which there is serious global action on climate change (worst case – RCP4.5 and best case – RCP 2.6). A simple way to think about them is as follows. RCP8.5 represents ‘business as usual’ – strong economic development for the rest of this century, driven primarily by dependence on fossil fuels. RCP6 represents a world with no global coordinated climate policy, but where lots of localized clean energy initiatives do manage to stabilize emissions by the latter half of the century. RCP4.5 represents a world that implements strong limits on fossil fuel emissions, such that greenhouse gas emissions peak by mid-century and then start to fall. RCP2.6 is a world in which emissions peak in the next few years, and then fall dramatically, so that the world becomes carbon neutral by about mid-century.

Note that in RCP2.6 the temperature does fall, after reaching a peak just below 2°C of warming over pre-industrial levels. That’s because RCP2.6 is a scenario in which concentrations of greenhouse gases in the atmosphere start to fall before the end of the century. This is only possible if we reduce global emissions so fast that we achieve carbon neutrality soon after mid-century, and then go carbon negative. By carbon negative, I mean that globally, each year, we remove more CO2 from the atmosphere than we add. Whether this is possible is an interesting question. But even if it is, the model results show there is no time within the next thousand years when it is anywhere near as cool as it is today.

(4) Most of the heat is going into the oceans


(Box 3.1 Fig 1) Plot of energy accumulation in ZJ (1 ZJ = 1021 J) within distinct components of Earth’s climate system relative to 1971 and from 1971–2010 unless otherwise indicated. See text for data sources. Ocean warming (heat content change) dominates, with the upper ocean (light blue, above 700 m) contributing more than the deep ocean (dark blue, below 700 m; including below 2000 m estimates starting from 1992). Ice melt (light grey; for glaciers and ice caps, Greenland and Antarctic ice sheet estimates starting from 1992, and Arctic sea ice estimate from 1979–2008); continental (land) warming (orange); and atmospheric warming (purple; estimate starting from 1979) make smaller contributions. Uncertainty in the ocean estimate also dominates the total uncertainty (dot-dashed lines about the error from all five components at 90% confidence intervals).


Note the relationship between this figure (which shows where the heat goes) and the figure I showed above that shows change in cumulative energy budget from different sources. Both graphs show zettajoules accumulating over about the same period (1970-2011). But the first graph has a cumulative total just short of 800ZJ by the end of the period, while this one shows the earth storing “only” about 300ZJ of this. Where did the remaining energy go? Because the earth’s temperature rose during this period, it also lost increasingly more energy back into space. When greenhouse gases trap heat, the earth’s temperature keeps rising until outgoing energy and incoming energy are in balance again.

(5) Current rates of ocean acidification are unprecedented.

The IPCC report says “The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. … It is virtually certain that the increased storage of carbon by the ocean will increase acidification in the future, continuing the observed trends of the past decades. … Estimates of future atmospheric and oceanic carbon dioxide concentrations indicate that, by the end of this century, the average surface ocean pH could be lower than it has been for more than 50 million years”.

(Fig SPM.7c) CMIP5 multi-model simulated time series from 1950 to 2100 for global mean ocean surface pH. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings

(Fig SPM.7c) CMIP5 multi-model simulated time series from 1950 to 2100 for global mean ocean surface pH. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Black (grey shading) is the modelled historical evolution using historical reconstructed forcings. [The numbers indicate the number of models used in each ensemble.]

Ocean acidification has sometimes been ignored in discussions about climate change, but it is a much simpler process, and is much easier to calculate (notice the uncertainty range on the graph above is much smaller than most of the other graphs). This graph shows the projected acidification in the best and worst case scenarios (RCP2.6 and RCP8.5). Recall that RCP8.5 is the “business as usual” future.

Note that this doesn’t mean the ocean will become acid. The ocean has always been slightly alkaline – well above the neutral value of pH7. So “acidification” refers to a drop in pH, rather than a drop below pH7. As this continues, the ocean becomes steadily less alkaline. Unfortunately, as the pH drops, the ocean stops being supersaturated for calcium carbonate. If it’s no longer supersaturated, anything made of calcium carbonate starts dissolving. Corals and shellfish can no longer form. If you kill these off, the entire ocean foodchain is affected. Here’s what the IPCC report says: “Surface waters are projected to become seasonally corrosive to aragonite in parts of the Arctic and in some coastal upwelling systems within a decade, and in parts of the Southern Ocean within 1–3 decades in most scenarios. Aragonite, a less stable form of calcium carbonate, undersaturation becomes widespread in these regions at atmospheric CO2 levels of 500–600 ppm”.

(6) We have to choose which future we want very soon.

In the previous IPCC reports, projections of future climate change were based on a set of scenarios that mapped out different ways in which human society might develop over the rest of this century, taking account of likely changes in population, economic development and technological innovation. However, none of the old scenarios took into account the impact of strong global efforts at climate mitigation. In other words, they all represented futures in which we don’t take serious action on climate change. For this report, the new “RCPs” have been chosen to allow us to explore the choice we face.

This chart sums it up nicely. If we do nothing about climate change, we’re choosing a path that will look most like RCP8.5. Recall that this is the one where emissions keep rising just as they have done throughout the 20th century. On the other hand, if we get serious about curbing emissions, we’ll end up in a future that’s probably somewhere between RCP2.6 and RCP4.5 (the two blue lines). All of these futures give us a much warmer planet. All of these futures will involve many challenges as we adapt to life on a warmer planet. But by curbing emissions soon, we can minimize this future warming.

(Fig 12.5) Time series of global annual mean surface air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean (solid lines) and the 5–95% range (±1.64 standard deviation) across the distribution of individual models (shading). Discontinuities at 2100 are due to different numbers of models performing the extension runs beyond the 21st century and have no physical meaning. Only one ensemble member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available.


Note also that the uncertainty range (the shaded region) is much bigger for RCP8.5 than it is for the other scenarios. The more the climate changes beyond what we’ve experienced in the recent past, the harder it is to predict what will happen. We tend to use the difference across different models as an indication of uncertainty (the coloured numbers shows how many different models participated in each experiment). But there’s also the possibility of “unknown unknowns” – surprises that aren’t in the models, so the uncertainty range is likely to be even bigger than this graph shows.

(7) To stay below 2°C of warming, the world must become carbon negative.

Only one of the four future scenarios (RCP2.6) shows us staying below the UN’s commitment to no more than 2ºC of warming. In RCP2.6, emissions peak soon (within the next decade or so), and then drop fast, under a stronger emissions reduction policy than anyone has ever proposed in international negotiations to date. For example, the post-Kyoto negotiations have looked at targets in the region of 80% reductions in emissions over say a 50 year period. In contrast, the chart below shows something far more ambitious: we need more than 100% emissions reductions. We need to become carbon negative:

(Figure 12.46) a) CO2 emissions for the RCP2.6 scenario (black) and three illustrative modified emission pathways leading to the same warming, b) global temperature change relative to preindustrial for the pathways shown in panel (a).


The graph on the left shows four possible CO2 emissions paths that would all deliver the RCP2.6 scenario, while the graph on the right shows the resulting temperature change for these four. They all give similar results for temperature change, but differ in how we go about reducing emissions. For example, the black curve shows CO2 emissions peaking by 2020 at a level barely above today’s, and then dropping steadily until emissions are below zero by about 2070. Two other curves show what happens if emissions peak higher and later: the eventual reduction has to happen much more steeply. The blue dashed curve offers an implausible scenario, so consider it a thought experiment: if we held emissions constant at today’s level, we have exactly 30 years left before we would have to instantly reduce emissions to zero forever.

Notice where the zero point is on the scale on that left-hand graph. Ignoring the unrealistic blue dashed curve, all of these pathways require the world to go net carbon negative sometime soon after mid-century. None of the emissions targets currently being discussed by any government anywhere in the world are sufficient to achieve this. We should be talking about how to become carbon negative.

One further detail. The graph above shows the temperature response staying well under 2°C for all four curves, although the uncertainty band reaches up to 2°C. But note that this analysis deals only with CO2. The other greenhouse gases have to be accounted for too, and together they push the temperature change right up to the 2°C threshold. There’s no margin for error.

(8) To stay below 2°C of warming, most fossil fuels must stay buried in the ground.

Perhaps the most profound advance since the previous IPCC report is a characterization of our global carbon budget. This is based on a finding that has emerged strongly from a number of studies in the last few years: the expected temperature change has a simple linear relationship with cumulative CO2 emissions since the beginning of the industrial era:



The chart is a little hard to follow, but the main idea should be clear: whichever experiment we carry out, the results tend to lie on a straight line on this graph. You do get a slightly different slope in one experiment, the “1%/yr” experiment, where only CO2 rises, and much more slowly than it has over the last few decades. All the more realistic scenarios lie in the orange band, and all have about the same slope.

This linear relationship is a useful insight, because it means that for any target ceiling for temperature rise (e.g. the UN’s commitment to not allow warming to rise more than 2°C above pre-industrial levels), we can easily determine a cumulative emissions budget that corresponds to that temperature. So that brings us to the most important paragraph in the entire report, which occurs towards the end of the summary for policymakers:


Unfortunately, this paragraph is a little hard to follow, perhaps because there was a major battle over the exact wording of it in the final few hours of inter-governmental review of the “Summary for Policymakers”. Several oil states objected to any language that put a fixed limit on our total carbon budget. The compromise was to give several different targets for different levels of risk. Let’s unpick them. First notice that the targets in the first sentence are based on looking at CO2 emissions alone; the lower targets in the second sentence take into account other greenhouse gases, and other earth systems feedbacks (e.g. release of methane from melting permafrost), and so are much lower. It’s these targets that really matter:

  • To give us a one third (33%) chance of staying below 2°C of warming over pre-industrial levels, we cannot ever emit more than 880 gigatonnes of Carbon. 
  • To give us a 50% chance, we cannot ever emit more than 840 gigatonnes of Carbon.
  • To give us a 66% chance, we cannot ever emit more than 800 gigatonnes of Carbon.

Since the beginning of industrialization, we have already emitted a little more than 500 gigatonnes. So our remaining budget is somewhere between 300 and 400 gigatonnes of carbon. Existing known fossil fuel reserves are enough to release at least 1000 gigatonnes. New discoveries and unconventional sources will likely more than double this. That leads to one inescapable conclusion:

Most of the remaining fossil fuel reserves must stay buried in the ground.

We’ve never done that before. There is no political or economic system anywhere in the world currently that can persuade an energy company to leave a valuable fossil fuel resource untapped. There is no government in the world that has demonstrated the ability to forgo the economic wealth from natural resource extraction, for the good of the planet as a whole. We’re lacking both the political will and the political institutions to achieve this. Finding a way to achieve this presents us with a challenge far bigger than we ever imagined.

Update (10 Oct 2013): An earlier version of this post omitted the phrase “To stay below 2°C of warming” from the last key point.



Two opposing cloud feedback loops












Other posts in this series, so far:

At the start of this series, I argued that Climate Science is inherently a Systems Discipline. To develop that idea, I described two important systems as feedback loops: the earth’s temperature equilibrium loop and economic growth and energy consumption, and then we put these two systems together.

The basic climate system now looks like this (leaving out, for now, the dynamics that drive economic development and energy use):

The basic planetary energy balancing loop, with the burning of fossil fuels forcing the temperature to change


Recall that the balancing loop (marked with a ‘B’) ensures that for each change to the input forcings (in this case greenhouse gases and aerosols in the atmosphere), the earth system will settle down to a new equilibrium point: a temperature at which the incoming and outgoing energy flows are balanced again. Each time we increase the concentration of greenhouse gases in the atmosphere, we can expect the earth to warm, slowly, until it reaches this new equilibrium. The economy-energy system (not shown above) is ensuring that we keep on adding more greenhouse gases, so we’re continually pushing the system further and further out of balance. That means we’re continually increasing the eventual temperature rise that the earth will experience before it reaches a new equilibrium.

Meanwhile, the aerosols provide a slight cooling effect, but they wash out of the atmosphere fairly quickly, so their overall concentration isn’t rising much. Carbon dioxide does not wash out quickly – it can remain in the atmosphere for thousands of years. Hence the warming effect dominates.

Now, if that was the whole picture, climate change would be very predictable, using basic thermodynamic principles. Unfortunately, there are other feedback loops that we haven’t considered yet. Here’s one:

The basic climate system with the ice albedo feedback

The basic climate system with the ice albedo feedback

As the temperature rises, the ice sheets start to melt and shrink. These include the Arctic sea ice, glaciers on Greenland and the Antarctic, and mountain glaciers across the world. When sea ice melts, it leaves more sea exposed, which is much darker than the ice. When land ice melts, it uncovers rocks, soils, and (eventually) plants, all of which are also darker than ice. Because of this, loss of ice leads to a lower albedo for the planet. A lower albedo means less of the incoming sunlight is reflected straight back into space, so more reaches the surface. In other words, less albedo means more incoming solar radiation. And, as we already know, this leads to more energy retained and more warming. In other words, it is a re-inforcing feedback loop.

As a quick check, we can use the rule of thumb that reinforcing loops have an even number of ‘-‘ links. Trace the path of this loop to check:

Ice albedo feedback loop on its own

Ice albedo feedback loop on its own

Because this is a reinforcing loop, it can modify the behaviour of the basic energy balancing loop. If a warming process starts, this loop can accelerate it, and cause more warming than we’d expect from just the main balancing loop. In extreme cases, a reinforcing loop can completely destabilize a system that is normally dominated by balancing loops. However, all reinforcing loops also must have limits (remember: nothing can grow forever). In this case, there is clearly a limit once all the ice sheets on the planet have melted. The loop can no longer function at that point.

Here’s another reinforcing loop:

Climate system with permafrost feedback

Climate system with permafrost feedback

In this loop, as the temperature rises, it melts the permafrost across Northern Canada and Russia. This releases the methane from the frozen soils. Methane is a greenhouse gas, so this loop also accelerates the warming. Again, it’s a re-inforcing loop, and again, there’s a limit: the loop must stop once all the permafrost has melted.

Here’s another:

Climate system with carbon sinks feedback

Climate system with carbon sinks feedback

This loop occurs because the more greenhouse gases we put into the atmosphere, the more work the carbon sinks have to do. Carbon sinks include the ocean and soils – they slowly remove carbon dioxide from the atmosphere. But the more carbon they have to absorb, the less effective they are at taking more. There’s an additional effect for the ocean, because a warmer ocean is less able to absorb CO2. Some model studies even suggest that after a few degrees of warming, the ocean might stop being a carbon sink and start being a source.

So, put that altogether and we have three re-inforcing loops working to destabilize the main energy balance loop. The main loop tends to limit the amount of warming we might expect, and the reinforcing loops all tend to increase it:

All three reinforcing loops working together

All three reinforcing loops working together

Remember, all three re-inforcing loops might operate at once. More likely, each will kick in at different times as the planet warms. Predicting when that might occur is hard, as is calculating the likely size of the effect. We can calculate absolute limits to each of these reinforcing loops, but there are likely to be other reasons why the loop stops working before reaching these absolute limits.

One of the goals of climate modelling is to capture these kinds of feedbacks in a computational model, to attempt to quantify the effects, so that we can understand them better. We can use both basic physics and empirical observations to put numbers on each of the relationships in the diagram, and we can experiment with the model to test how sensitive it is to different kinds of perturbation, especially in areas where it’s hard to be sure about the numbers.

However, there’s also the possibility that we missed some important feedback loops. In the model above, we have missed an important one, to do with clouds. We’ll meet that in the next post…

Other posts in this series, so far:



Effect of two different kinds of pollutant

Effect of two different kinds of pollutant




The effect of these two different kinds of pollutant

The effect of these two different kinds of pollutant


The energy system interacting with the basic climate system

The energy system interacting with the basic climate system











Other posts in this series, so far: