Day 2: Royal Society Meeting (ICCP-AR5)

The talks on the second day were very interesting and were mainly about the status of the science, future challenges and uncertainties. The earth’s climate is just inherently complex . These notes are also partly for my benefit !

1. Clouds : David Randall

Clouds are hard to model. They scatter and emit raditaion. They also transport  energy, moisture and momentum large distances. Cloud models go back to the 60s but have remained very much at the macro scale and this is one of the main problems because cloud formation depends more on micro-physics. Cloud seeding is from natural and anthropogenic aerosols and models lack resolution at these scales.

Globally the net cooling effect of clouds is about -20 W/m2. The water content of clouds is 100 times smaller than the water vapour content of the atmosphere, yet their effect on climate is huge. Large  convective  rain clouds tend to be on average radiation neutral while low clouds are strongly cooling and can be caused by sinking air from convective areas. Cirrus clouds have a net warming effect.  CMIP5 models have an average positive cloud feedback of  ~ 0.6 w/m2/degC. There are two arguments for a net positive feedback.

  1. Anvil hypothesis: Tropical anvil convective clouds flatten off at a fixed temperature of 200K so emit the same IR independent of surface temperature. Therefore as surface temperature rises IR radiation doesn’t – a net positive feedback .
  2. Low clouds diminish with warner temperatures ( low confidence).

It is entirely possible that cloud feedback is in fact negative and this is the largest current uncertainty of GCMs.

2. Aerosols: Olivier Boucher

Aerosols have 3 main effects:

  1. They scatter incoming solar radiation cooling the earth.
  2. They (e.g. black carbon) absorb both incoming solar radiation and surface IR radiation
  3. They help seed clouds formation – net cooling effect.

Energy imbalance Q = F -\lambda\Delta{T}  where \lambda is the aerosol feedback.

Models trade off aerosols against Climate Sensitivity to match observed temperatures. Aerosols are essentially the tuning parameter that match GCMs in hindcasts to previous surface temperatures.

Globally 20-40% of aerosol optical depth is of anthropogenic origin. Somewhere between 1/4 to 2/3 of cloud condensation is on nuclei concentrations of anthropogenic origin. The amount of black carbon emitted in Asia is underestimated but probably overestimated elsewhere. One question that occurred to me was whether we know if the early 19th century temperature measurements in Europe were suppressed due to the wide-scale burning of coal both in households and industry.  Is some of the warming after 1956 not actually due to the clean air acts across Europe ?  Weather station data were concentrated in Europe and N. America before the early 20th century.  All CMIP5 models use the same historic aerosol trends. Just how well are these trends really known ?

There is no correlation of cloud cover with cosmic rays. The statement for the Hiatus was that 50% of the pause can be explained by natural variation ( 1/3 volcanic, 2/3 solar). The other 50% is of unknown origin !

Aerosol-clouds are estimated  as a negative feedback   ~-0.45 Wm-2deg.C-1. The total effective radiative forcing due to aerosols is ~ -0.9 Wm-2.

3. Carbon + Geochemical Cycles: Corinne le Quere

CO2 is 40% above pre-industrial levels.  An extra 180 Gtons of Carbon has been added to atmosphere. Sources of atmospheric CO2 emissions which are currently running at 10 Gtons/year :

  1. Deforestation =   2.9 Gtons/year    regrowth = 1.3 Gtons/year
  2. Emissions  is 2/3 of rise or 6.6 Gtons/year
  3. Land use is 1/3 of rise or 3.3 Gtons/year

4. Weather extremes: Denns Hartman

The AR5 assessment plays down the risk of climate catastrophes. Increases in extremes are based on

  • warming is small ~0.6C since 1950
  • Corresponds to a 4% change in saturated vapour pressure (C-C equation)
  • making statements about changes in extremes is very difficult but studies show (based on datasets HADEX2  HADGHCND ) that   warm nights have increased    4.5 ± 0.9 % and warm days increased by 3± 1.8 %

Weak statements can be made about precipitation. In general wet areas get wetter and dry areas get dryer. There is little evidence that droughts have got worse. There is also low confidence that flooding also has got worse.

  • Tropical Storms: No significant change
  • Intensity of storms: No significant change.

5. Model confidence : Peter Stott

Very much a party line talk this one. Humans are the “dominant” cause of warming since the mid-20th century. It is extremely likely(95% confidence) that more than HALF of the observed warming is anthropogenic. See table 10.1 for justification for this statement. A summary of forcing causes is shown in Figure 10.5 below.

Fig 10.5 from AR5. ANT is the net anthropogenic forcing. I do not understand how the ANT errors get smaller after adding GHG and OA together !

Fig 10.5 from AR5. ANT is the net anthropogenic forcing. I do not understand how the ANT errors get smaller after adding GHG and OA together !

Presumably then up to HALF of the observed warming  could also be natural.  He accepts that solar output and volcanic aerosols are forcing agents which can explain half the hiatus, but he dismisses natural internal variability (PDO, ENSP, AMO etc) as being only of order ± 0.1 W/m2.

Kenneth Trenberth then asks the question:  Given the observed 15 year hiatus in global warming how can  natural forcings be just zero ± 0.1 ?  Why is ENSO not included in assessments ?

Peter Stott gave a hand waving non-answer to this point. Has Trenberth become a skeptic ?

6. Circulation: Ted Shepherd

His point is that all GCM models really deal with is energy balance locally. They do not really handle circulation at all and exhibit severe biases. The importance of dynamic circulation is the horizontal transport of moisture, energy and momentum. A severe limitation is  understanding the dynamic circulation aspect of climate.

  • North Atlantic Oscillation is natural and so is are other internal variability.
  • Hadley cell has been widening but it narrows during el Nino.
  • WCRP grand challenge on Clouds, Circulation and Climate Sensitivity.
  • CMIP5 are not true ensembles and probability distribution funstions do not apply

7. Paleoclimate models: Gavin Schmidt

CMIP5 models have been applied to understanding climates of the past. This means using the known orbital parameters, CO2 , Ice cover , sea-level that were prevalent then.

2 examples:

  • Tier 2: Mid-holocene (6000 – LGM 21 K years ago)
  • Tier 3 last Millenium

He claims that overall they work well but there are model dependent divergences in Sahel, SW US etc.

However he admits that none of the models are able to reproduce the on-set of an Ice Age or predict the next Ice Age ! They are static representations rather than dynamic.

8. Politics & Propaganda: John Ashton

I won’t say much about this as I already commented on the politics in the previous post.

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6 Responses to Day 2: Royal Society Meeting (ICCP-AR5)

  1. If clouds have a net cooling effect of -20Wm-2, wouldn’t that indicate a net negative feedback?

    • Clive Best says:

      Certainly clouds cool the planet and 68% of the surface is covered in clouds. IPCC assume that clouds only react to other external forcing changes. They say that clouds are a feedback and that if surface temperature rises so the amount of low clouds will decrease resulting in a bit of extra warming. This (plus the anvil cloud hypothesis) is a positive feedback which models estimate as 0.6 W/m2/deg.C

      However, I think that clouds are themselves an external forcing which directly effects the surface temperature. For example we know that el Nino changes cloud cover. If cloud cover falls by 2% the “CRE” effect is nearly 0.5 W/m2 which warms the surface. In other words clouds are decoupled from CO2.

  2. Ken Gregory says:

    Hockey Schtick:
    No, this statement tells us nothing about how clouds change when temperatures change. The statement only says at today’s global average temperature, clouds have a net cooling effect of -20 W/m^2 compared to no clouds. The cloud feedback answers the question, if temperatures increase by one degree Celsius but no other factors (water vapor, lapse rate, surface albedo, ocean and air circulation patterns) change, how much do temperatures change.

    A direct regression of temperature and cloud amount (or cloud radiative effect) tells us nothing about the cloud feedback because the data includes both feedback (temperatures causing cloud changes) and radiative forcing (other factors causing cloud changes that cause temperature changes). It is very difficult to separate the two effects. Spencer’s papers proved that if you falsely assume that clouds change only in response to temperature changes (ignoring radiative forcing changes by clouds), your regression will always overestimate cloud feedback.

  3. Ken Gregory says:

    The statement for the Hiatus was that 50% of the pause can be explained by natural variation ( 1/3 volcanic, 2/3 solar).

    The IPCC assumes the sun can affect climate only by changes in the solar irradiance energy. The IPCC has been saying for decades that the sun has insignificant effect on climate. Figure SPM.5 shows natural forcings since 1750 (0.05 W/m2) is only 2% of total forcing (0.05 + 2.29 W/m2). Now they claim that tiny changes in the solar irradiance has counteracted the warming effects of CO2 over the last 15 years when emissions are at high levels, but the sun didn’t affect climate before 1998.

    There were five large volcanoes during the period 1969 to 1995 that caused significant aerosol cooling, but no volcanoes since 1992 that could have caused a cooling effect. The IPCC appears to have just fabricated this volcanic cooling claim. The satellite data refutes it.

    NASA publishes satellite data that show a large reduction in the amount of volcanic aerosols during the recent period. Blogger Lucia Liljegren reports that the average forcing from the lack of volcanic eruption during 1998 to 2012 is 0.28 W/m2 more than the period 1951 to 2012 as shown in the graph:

    The TSI solar forcing is a trivial 0.018 W/m2 less during 1998 to 2012 than 1951 to 2012. The volcanic plus solar forcing is 0.26 W/m2 greater in the recent period, which would have cause increased warming, not a pause.

  4. Josh says:

    Hi, Clive, great notes. I did my own rather less scientific cartoon notes (over on BishopHill) and wish I had known you were there and got a chance to meet you!

    I got here via your comment on Fixing the facts 2 at Climate Audit. I am still trying to puzzle out exactly how much the IPCC think the 1950 – 2010 warming is anthropogenic and how much is natural.

    If you can help then do email me josh at

    • Clive Best says:


      Your cartoons are just great and I actually think they are also appreciated even by many mainstream climate scientists. I particularly enjoyed your portrayal of the Royal Society talks.

      I am convinced that privately most climate scientists would probably now admit that somewhere between 20-50% of the warming observed between 1950-2010 was natural. There is no other rational way to explain the current hiatus or what caused the cool period after 1940 – 1970. The main problem with admitting this is that it translates to the IPCC having over-exaggerating warming by somewhere between a factor of 1.5 to 2.

      The obstacle is the political exploitation of science for ulterior motives. It will take 10 years to stop this juggernaut – that is assuming it can be stopped before rationality eventually prevails.


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