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How sensitive is our climate?
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Net positive feedback is confirmed by many different lines of evidence. |
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Climate sensitivity is low
"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)
At-a-glance
Climate sensitivity is of the utmost importance. Why? Because it is the factor that determines how much the planet will warm up due to our greenhouse gas emissions. The first calculation of climate sensitivity was done by Swedish scientist Svante Arrhenius in 1896. He worked out that a doubling of the concentration of CO2 in air would cause a warming of 4-6oC. However, CO2 emissions at the time were miniscule compared to today's. Arrhenius could not have foreseen the 44,250,000,000 tons we emitted in 2019 alone, through energy/industry plus land use change, according to the IPCC Sixth Assessment Report (AR6) of 2022.
Our CO2 emissions build up in our atmosphere trapping more heat, but the effect is not instant. Temperatures take some time to fully respond. All natural systems always head towards physical equilibrium but that takes time. The absolute climate sensitivity value is therefore termed 'equilibrium climate sensitivity' to emphasise this.
Climate sensitivity has always been expressed as a range. The latest estimate, according to AR6, has a 'very likely' range of 2-5oC. Narrowing it down even further is difficult for a number of reasons. Let's look at some of them.
To understand the future, we need to look at what has already happened on Earth. For that, we have the observational data going back to just before Arrhenius' time and we also have the geological record, something we understand in ever more detail.
For the future, we also need to take feedbacks into account. Feedbacks are the responses of other parts of the climate system to rising temperatures. For example, as the world warms up. more water vapour enters the atmosphere due to enhanced evaporation. Since water vapour is a potent greenhouse gas, that pushes the system further in the warming direction. We know that happens, not only from basic physics but because we can see it happening. Some other feedbacks happen at a slower pace, such as CO2 and methane release as permafrost melts. We know that's happening, but we've yet to get a full handle on it.
Other factors serve to speed up or slow down the rate of warming from year to year. The El Nino-La Nina Southern Oscillation, an irregular cycle that raises or lowers global temperatures, is one well-known example. Significant volcanic activity occurs on an irregular basis but can sometimes have major impacts. A very large explosive eruption can load the atmosphere with aerosols such as tiny droplets of sulphuric acid and these have a cooling effect, albeit only for a few years.
These examples alone show why climate change is always discussed in multi-decadal terms. When you stand back from all that noise and look at the bigger picture, the trend-line is relentlessly heading upwards. Since 1880, global temperatures have already gone up by more than 1oC - almost 2oF, thus making a mockery of the 2010 Monckton quote in the orange box above.
That amount of temperature rise in just over a century suggests that the climate is highly sensitive to human CO2 emissions. So far, we have increased the atmospheric concentration of CO2 by 50%, from 280 to 420 ppm, since 1880. Furthermore, since 1981, temperature has risen by around 0.18oC per decade. So we're bearing down on the IPCC 'very likely' range of 2-5oC with a vengeance.
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Further details
Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we manage, against all the good advice, to double the amount of carbon dioxide in the atmosphere. This includes feedbacks that can either amplify or dampen the warming. If climate sensitivity is low, as some climate 'skeptics' claim (without evidence), then the planet will warm slowly and we will have more time to react and adapt. If sensitivity is high, then we could be in for a very bad time indeed. Feeling lucky? Let's explore.
Sensitivity is expressed as the range of temperature increases that we can expect to find ourselves within, once the system has come to equilibrium with that CO2 doubling: it is therefore often referred to as Equilibrium Climate Sensitivity, hereafter referred to as ECS.
There are two ways of working out the value of climate sensitivity, used in combination. One involves modelling, the other calculates the figure directly from physical evidence, by looking at climate changes in the distant past, as recorded for example in ice-cores, in marine sediments and numerous other data-sources.
The first modern estimates of climate sensitivity came from climate models. In the 1979 Charney report, available here, two models from Suki Manabe and Jim Hansen estimated a sensitivity range between 1.5 to 4.5°C. Not bad, as we will see. Since then further attempts at modelling this value have arrived at broadly similar figures, although the maximum values in some cases have been high outliers compared to modern estimates. For example Knutti et al. 2006 entered different sensitivities into their models and then compared the models with observed seasonal responses to get a climate sensitivity range of 1.5 to 6.5°C - with 3 to 3.5°C most likely.
Studies that calculate climate sensitivity directly from empirical observations, independent of models, began a little more recently. Lorius et al. 1990 examined Vostok ice core data and calculated a range of 3 to 4°C. Hansen et al. 1993 looked at the last 20,000 years when the last ice age ended and empirically calculated a climate sensitivity of 3 ± 1°C. Other studies have resulted in similar values although given the amount of recent warming, some of their lower bounds are probably too low. More recent studies have generated values that are more broadly consistent with modelling and indicative of a high level of understanding of the processes involved.
More recently, and based on multiple lines of evidence, according to the IPCC Sixth Assessment Report (2021), the "best estimate of ECS is 3°C, the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C. It is virtually certain that ECS is larger than 1.5°C". This is unsurprising since just a 50% rise in CO2 concentrations since 1880, mostly in the past few decades, has already produced over 1°C of warming. Substantial advances have been made since the Fifth Assessment Report in quantifying ECS, "based on feedback process understanding, the instrumental record, paleoclimates and emergent constraints". Although all the lines of evidence rule out ECS values below 1.5°C, it is not yet possible to rule out ECS values above 5°C. Therefore, in the strictly-defined IPCC terminology, the 5°C upper end of the very likely range is assessed to have medium confidence and the other bounds have high confidence.
Fig. 1: Left: schematic likelihood distribution consistent with the IPCC AR6 assessments that equilibrium climate sensitivity (ECS) is likely in the range 2.5°C to 4.0°C, and very likely between 2.0°C and 5.0°C. ECS values outside the assessed very likely range are designated low-likelihood outcomes in this example (light grey). Middle and right-hand columns: additional risks due to climate change for 2020 to 2090. Source: IPCC AR6 WGI Chapter 6 Figure 1-16.
It’s all a matter of degree
All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4°C or even more. These are not small rises: they would signal many damaging and highly disruptive changes to the environment (fig. 1). In this light, the arguments against reducing greenhouse gas emissions because of "low" climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being that as a consequence, we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see. Both such stances are nothing short of stupid. Inaction or complacency in the face of the evidence outlined above severely heightens risk. It is gambling with the entire future ecology of the planet and the welfare of everyone on it, on the rapidly diminishing off-chance of being right.
Last updated on 12 November 2023 by John Mason. View Archives
You will notice that that small spike noted above has become a deep trough that overlaps with the first trough, with a resulting large reduction in the atmospheric window. But that widening took place step by step, and in each step, it was always the smallest effect.
Clearly, if you looked up at 680 cm^-1 wave number, all you would see is the thermal radiation of the atmosphere. You would not even be able to see the sun in that portion of the spectrum, from the surface. In contrast, the intervals between 810 and 950 probably have sufficiently high transmittance to be useful for telescopes (and sidewinders). That is a atmospheric window. There is another, smaller one on the other side of the O3 trough.
With that in mind, closing the window means reducing the IR radiation from the surface that escapes to space, particularly in those parts of the spectrum; and it is a minor effect. Deepening of the CO2 emission band means that in that part of the spectrum outside of the atmospheric window, the amount of IR from the atmosphere itself is reduced because it comes from a higher altitude and hence has a colder temperature.
These effects are not strictly independent. For example, at the right edge of the CO2 trough, there are transmittances that rise from around 0.2 to 0.8 over a 60 wave number interval. Over this interval, both trough deepening,and narrowing of the atmospheric window occur with rising CO2. On the equivalent left side, however, transmittances peak around 0.2 because of the overlapping effects of H2O. The trough deepening, however, contributes almost as much to the reduced OLR as does the equivalent on the other side.
Speaking of which, on the graph shown, total area under the line equals the total power (watt's per square meter) radiated to space, so the difference in area is the difference in radiated power. As you can see, the most significant part of this comes from the deepening of the trough on the wings, and that is approximately equal on both sides, even where transmittances are very low. Therefore it is clearly not a narrowing of the atmospheric window.
Although this only indicates transmittance in one direction, be assured it is calculated in both. In the thread from which this comes SoD is developing a simple radiation model, and you can see in the code that he makes the calculations first for upwelling, and then for downwelling radiation.
(By the way, this illustration also appears in SoD's thread on theory and experiment in atmospheric radiation, from which I got the diagrams which showed the close correlation between the model predictions and observations. That thread has already been linked here. So your claim that all you have received is statements, not evidence, is nonsense.)
Because the transfers in radiation are calculated for each wavenumber, and for each level independently, there is no single calculation that corresponds to what you are seeking, ie, a level in which all incoming radiation is from the surface, and all upwelling radiation goes to space. But that does not mean that both the upwelling and downwelling emittance from each level is ignored, or that the absorption at any level is ignored which is what is required for George White's adjustment to make any sense.
Of course, in the LBL models, the total upwelling radiation of the highest level (emitted and transmitted) is just the Outgoing Long-wave Radiation. And the difference between that for 375 ppm and for 750 ppm is the increase of the greenhouse effect for doubling of CO2.
So, if you want to verify Modtran, and all the other LBL models programed by different teams around the world, and all the energy balance models also programed by different teams around the world, which all come up with essentially the same result; which just happens to match observations almost exactly, you either need to accept the observational match as confirming the models, or you need to go through the models line by line. There is no other short cut.
Climate Myth...