Climate Science Should Take Account of New Findings Since the IPCC’s Sixth Assessment

I have long regarded myself as a climate sceptic.  I am definitely not an expert on climate science: it is a topic I have learnt a little about as an accompaniment to my studies in environmental economics.  The literature is vast, and I have not read more than a tiny proportion.  I am in the predicament of everyone who tries to arrive at sensible opinions on a scientific subject on whch they lack expertise: all I can do is make the best judgments I can based on a partial understanding, on perceptions of the trustworthiness of experts and of the institutions within they operate, and on an understanding of the nature and limits of scientific knowledge. 

I know enough to recognise that the influences on climate, both natural and human-influenced, are numerous and complex – far more so than many people appreciate -, and that our knowledge of those influences is imperfect.  The IPCC’s assessment reports, to its credit, are full of statements that are followed by italicized qualifications, some implying a fair degree of  uncertainty, such as ‘likely’ and ‘medium confidence’.  Many of those statements, moreover, are subject to multiple qualifications in the form of ranges of possible values combined with verbal qualifications.  Consider for example the following statement on equilibrium climate sensitivity (ECS), a key quantity in climate science (of which more below):

In summary, based on multiple lines of evidence the best estimate of ECS is 3°C, it is likely within the range 2.5 to 4 °C and very likely within the range 2 to 5 °C. It is virtually certain that ECS is larger than 1.5°C. Whereas there is high confidence based on mounting evidence that supports the best estimate, likely range and very likely lower end, a higher ECS than 5°C cannot be ruled out, hence there is medium confidence in the upper end of the very likely range.  (1)

It is a strength of the IPCC that its reports synthesize the thinking of a very large number of scientists, but at the same time it is hard for an outsider to avoid the suspicion that it may be subject to an element of groupthink and slowness to adapt to new evidence.  Given that the IPCC is a body of the United Nations, it would not be surprising if its high level reports were subject to a degree of politically-motivated editing.  A final reason for scepticism is grounded in philosophy of science.  Unlike propositions of mathematics and logic, scientific theories are never conclusively proven.  However well-supported they may be, they always remain subject to the possibility that they may have to be abandoned or modified in the light of new evidence.  That applies to the theories of climate science just as much as to theories in other scientific fields. 

For these reasons, I have long considered it possible that climate change is not as serious a problem as we are told by the IPCC and others.  But here’s the thing.  None of the above considerations point in only one direction.  Just as they suggest that climate change might not be as serious a problem as we are told, they also suggest that it might be an even more serious problem with future warming faster than suggested by mainstream current thinking.  I am a sceptic in the sense of embracing both those possibilities, a balanced sceptic you might say.  And I have been rather uncomfortable about taking the latest  IPCC assessment as the fulcrum about which my scepticism is balanced.

At the time of writing, there is a specific reason for thinking that future warming may be faster than suggested by the latest IPCC assessment , AR6.  The main AR6 reports were published in 2021 and 2022, the Synthesis Report published in 2023 being based on those reports (IPCC).  Thus they did not take account of climate experience in 2023 and 2024 in which global temperatures were significantly higher than expected (2). 

Some papers I have read recently have also shifted my thinking somewhat in the ‘even more serious’ direction.  They relate to two important influences on climate: forests and aerosols. 

Forests: the Effect of Albedo

The planting of trees is widely considered to have the potential to make an important contribution to mitigating climate change (in addition to other benefits it can provide). It is sometimes suggested that any tree planting is good.  Wiser commentators emphasize the need for ‘the right tree in the right place’.  But what is the right place?  The IPCC says this:

[Afforestation and reforestation] activities may change the surface albedo and evapotranspiration regimes, producing net cooling in the tropical and subtropical latitudes for local and global climate and net warming at high latitudes (3)

Here albedo is the fraction of sunlight falling on an area of land that is reflected back into space.  Forests, because they are dark, tend to have low albedo and so absorb more solar energy than many other types of land use, an effect which can contribute to global warming.  Evapotranspiration is the transfer of water to the atmosphere from soil and plants: it can also influence climate but will not be further considered here. 

Recognition of the importance of albedo is not new, but it seems that only recently has the net effect on climate of carbon sequestration by forests and albedo been explored in fine spatial detail.  Hasler et al (2024) present a world map showing what they term the ‘net albedo effect’ of forest restoration by location (4). The net albedo effect is the percentage by which the warming effect of the lowering of albedo offsets the cooling effect of the increase in carbon sequetration.

As expected, the map shows a low offset (less than 25%) in much of the tropics, and a very high offset (more than 100%) in northern Canada and northern Siberia.  However, many other regions do not fit the pattern suggested by the IPCC quotation.  Regions with very high offsets are determined as much by dryness as by latitude, and include much of central Asia and North America, a belt across Africa south of the Sahara, the Horn of Africa, and considerable parts of south-western Africa, southern South America and Australia.  On the other hand regions at moderately high latitudes may still have low offsets if their rainfall is adequate: such regions include the eastern fringe of the US, much of western Europe including Britain, and much of east Asia.  An obvious explanation for this pattern is that both low temperatures and dryness result in slower growth and consequently less carbon sequestration.

These findings imply a considerable narrowing of ‘the right place’ to plant trees in order to mitigate global warming.  To give one instance, an IPCC report includes a case study involving the regeneration of 200 million trees in the Sahel region of Africa, clearly presented as an example of climate change mitigation (5).  However, the focus of the cited source for that figure (6) is the rebuilding of ecological and economic resilience by a rural population that had previously suffered drought and famine, and there is no mention of climate change mitigation.  The case study also cites (7) an estimate of the potential carbon sequestration of the region at a rate per hectare which it describes as ‘relatively modest’.  Neither the case study nor either of these sources give any consideration to the offsetting effect of the trees in lowering albedo.  Moreover the specific location, the Maradi/Zinder region of Niger, is clearly within the belt identified by Hasler et al in which the warming effect of albedo offsets the cooling effect of carbon sequestration by more than 100%, so (whatever their other benefits) the net effect of these 200 million trees may well be harmful so far as climate change mitigation is concerned.

Globally, the IPCC’s central estimate of the climate change mitigation potential of afforestation and reforestation (together with peatland and coastal wetland restoration) is 5 GtCO₂e per year (8).  It is unclear whether this figure allows for albedo offset at all (expressed in CO₂ equivalent terms), but if it does it is presumably based on the assumption, implicit in the IPCC quotation above, that albedo offset only results in net warming at high latitudes.  Given the findings of Hasler et al, the figure is likely to be a significant over-estimate.

It may have occurred to readers that there is potentially a silver lining to these unwelcome findings regarding the net albedo effect.  Could it be that the climate impact of deforestation in those regions where the albedo offset is high is much less serious than we have thought, because the loss of carbon sequetration is wholly or largely offset by a raising of albedo?  I think there is something in this, although it is not a claim made by Hasler et al.  But we should not exaggerate its significance.  Firstly, most of the world’s deforestation is occurring in the tropical regions of Latin America and South-East Asia (9), most of it in regions where, according to Hasler et al, albedo offset is low.  Secondly, even where deforestion is in regions where albedo offset is high, the impact on climate will depend very much on the circumstances.  If deforestation results from wildfire, combustion of the trees will result in the sudden release of carbon sequestered over many years, the warming effect of which will greatly outweigh any benefit from higher albedo of the new landscape. Much the same will apply if trees are cut for fuel, as is common in Africa.  A scenario in which deforestation might do little or no harm from a climate perspective would be the following: the trees are logged for timber and the timber is used in the production of durable goods such as buildings or furniture, so that the carbon remains sequestered; and the land is converted to a use with a higher albedo than the original forest, such as grassland and some types of cropland.  Even then, to anticipate slightly a point that will be developed below, much will depend on how much carbon is released from the soil during land conversion.

On balance the findings of Hasler et al appear to represent bad news in terms of climate change mitigation, because they imply that what has been understood to be promising means of mitigation, namely afforestation and reforestation, are in many regions of the world less effective than had been thought.

Forests: Other Considerations

The findings of a study by Baggio-Compagnucci et al (10) appears to represent further bad news.  This study focuses on the climate change mitigation potential of afforestation in Scotland, but offers lessons of wider relevance.  Reading from Hasler et al’s map, most of Scotland has an albedo offset of below 50% and so, in terms of carbon sequestration from tree growth and offsetting lowering of albedo, would be expected to be a suitable region for afforestation as a means of climate change mitigation.  Baggio-Compagnucci et al, however, consider some factors which Hasler et al ignore: the impact on tree growth of future climate change; the type of soil in which young trees are planted; and  the soil preparation process prior to planting.

When afforestation is undertaken as a means of climate change mitigation, the aim is to yield the maximum stream of benefits in the form of carbon sequestration over the many years of the trees’ lives.  Given the expectation that the climate will grow warmer over those years, the ideal tree species may not be the one best suited to the current climate.  Northern species such as birch may become less suitable (because less productive and so sequestering less carbon) as the climate warms, while more southern species such as oak may become more suitable (11).  But we cannot predict the path of future warming with any certainty.  The choice of species therefore involves an unavoidable element of risk.  Even if allowance is made for future warming based on the best information available at the time of planting, there will be a risk that the choice of species will prove less suitable than it might have been because the path of warming turns out to differ from expectations.  In concluding their discussion of species selection, Baggio-Compagnucci et al state as follows (12):

A forward-looking policy, aiming to enhance environmental resilience, would therefore use a mixture of species likely to succeed based on their present and future projected suitability.

Using a mixture of species would be, in my terminology, a risk averse strategy, unlikely to maximise carbon sequestration if the path of warming accords with expectations, but also unlikely to result in an outcome with carbon sequestration far below the maximum if the path differs from expectations. 

Which tree species are potentially suitable will vary between different regions of the world, but risk in respect of choice of species arising from the uncertain path of future warming can apply in many regions.  The global potential for climate change mitigation via afforestation, which is in any case limited by the availability of suitable land and competition with other important land uses such as food production, is also limited by the possibility that in some regions the species chosen will turn out not to be the most suitable for the actual path of warming. It is also as we have seen limited by albedo offset, a factor not mentioned by Baggio-Compagnucci et al (it would seem important to consider also how albedo might differ between tree species). 

Baggio-Compagnucci et al also found that the release of carbon from soil during preparation for tree planting can have a significant bearing on whether afforestation by a particular species at a particular location will contribute to climate change mitigation.  I felt that this part of their study could have been more clearly explained: how exactly the one-off release of carbon during soil preparation was weighed against the subsequent stream of annual carbon sequestration during tree growth was not apparent. 

Nevertheless, two points were clear.  Firstly, soils differ in their carbon content, with generally more carbon, and therefore more release of carbon during soil preparation, at higher altitudes (13).  Unfortunately it is higher altitude locations that tend to be preferred for afforestation because they are less desirable for other land uses, but tree productivity (and therefore carbon sequestration from tree growth) tends to be lower at higher altitudes (14).  Secondly, the amount of carbon released during soil preparation depends on the method of preparation, with a net benefit in terms of climate change mitigation most likely when a low intensity (ie hand (15)) method is used (16).

Baggio-Compagnucci et al conclude that, as a means of climate change mitigation, much of the Scottish uplands would be unsuitable for afforestation given intensive soil preparation methods such as ploughing (17). 

Taken together, the findings of Hasler et al and Baggio-Compagnucci et al imply that reforestation and afforestation can contribute to climate change mitigation only at locations, and with tree species, such that there is a net benefit after allowing for albedo offset and the effect of future warming on productivity and loss of carbon during soil preparation.  They also imply that in many locations and circumstances their net effect will be to worsen climate change.  That is a very long way from the common assumption that almost any tree planting is a climate benefit, and quite a long way from the IPCC quotation above suggesting that only latitude matters.

Equilibrium Climate Sensitivity

Before moving on to aerosols, it will be helpful to introduce the concept of equilibrium climate sensitivity. This is defined as the amount by which long-term average global surface air temperature increases in response to a doubling of atmospheric CO₂ concentration. The doubling is understood to be relative to the pre-industrial level, and long-term temperature is the temperature after feedback effects have had time to work through so that the global system is in equilbrium (18).  Of course we are also interested in how temperature will increase in response to less than doubling of CO₂ concentration, and in shorter times than are needed to reach equilibrium.  But equilibrium climate sensitivity (ECS) has become accepted as a standard measure of the responsiveness of temperature to increases in CO₂ concentration, facilitating comparison of the results of different researchers and providing a basis for predicting the path of future temperature in response to different possible circumstances. 

As the earlier IPCC quotation indicates, the value of ECS is not known with accuracy.  Different methods of estimation can yield different results, and every result comes with its own estimated range of uncertainty (19).  The IPCC’s best estimate of 3°C is towards the middle of the estimates of a large number of studies using a variety of methods. 

Methods of estimating ECS which start from the path of average temperature over any time period must address the problem of adjusting the path to exclude the effect of influences other than CO₂.  These other influences include short-lived greenhouse gases such as methane, the short-term effects of major volcanic eruptions, and various kinds of aerosols.

The Effect of Aerosols

Atmospheric aerosols are suspensions of very small particles.  Their chemical composition is various as are their sources, although many are produced, directly or indirectly, by the combustion of fossil fuels (20).  Most have a cooling effect on climate because they either scatter incoming sunlight themselves or act as seeds for clouds which scatter sunlight (although some absorb sunlight and so have a warming effect). This cooling effect depends partly on the albedo of Earth at surface level.  If the surface albedo at a certain location is high, then much of the sunlight heading for that location is going to be reflected back anyway, so any scattering by aerosols above will not make too much difference to the overall outcome.  It is when the surface albedo is low that scattering by aerosols has the most cooling effect.  Overall albedo, resulting from the combined effect of reflection at the surface and in the atmosphere, has been monitored using satellites by NASA’s CERES project yielding data both for Earth as a whole and at fine spatial scale (21).

There has been considerable uncertainty as to the overall size of the cooling effect.  That in turn is a major reason for uncertainty as to the size of ECS.  If the cooling effect of current and past levels of aerosols is low, then ECS is low, but if the cooling effect is high, so that much of the warming effect of CO₂ is masked by the effect of aerosols, then ECS is high (22). 

Hansen et al (2025) bring new evidence to bear on this problem.  It is appropriate to advise the reader that their paper is not presented in the normal style of an academic paper on a scientific topic.  The introduction, for example, describes aerosol masking of warming from greenhouse gases as a Faustian bargain (23), and includes an illustration of Faustus with Mephistopheles to emphasize the point.  There is a lengthy and opinionated epilogue ranging over topics not closely related to climate science.  None of this adds credibility to the paper as a report of a piece of scientific research.

Nevertheless, the paper does have a scientific core, starting from a decision by the International Maritime Organization to reduce the limit on sulphur in shipping fuel from 1 January 2020 from 3.5% to 0.5% (24).  This has provided a natural experiment offering new evidence of the size of the cooling effect of aerosols.  Compliance with the new limits appears to have been high, and therefore the level of atmospheric sulphate aerosols from shipping experienced a sudden reduction around that date.  Furthermore the reduction was not spatially uniform: it was particularly significant in the North Pacific and North Atlantic regions where the volume of shipping is high and where the atmosphere is relatively unpolluted from sources other than shipping.  The latter point is significant because it has been found that the cloud-seeding effect of aerosols is greater in pristine air than in air that is already polluted, a phenomenon known as non-linearity (25).  It would therefore be expected that the warming effect of removing a source of aerosols would be greater in a region with no other significant source of aerosls.

The following is an outline of the argument of Hansen et al.  The authors present data from CERES showing the Earth’s albedo falling between 2010 and 2024, with fluctuations about the downward trend, by about 0.5%.  This data does not prima facie suggest a sudden change in 2020, but the authors claim that after allowance is made for other influences on albedo – notably the Pacific Decadal Oscillation, a phenomena related to El Nino – such a sudden change is clear (26). 

Since the average sunlight reaching the Earth at the top of the atmosphere is 340 Wm^-2 (27) a reduction in albedo of 0.5% implies an increase in energy absorbed (forcing in climate science jargon) of 1.7 Wm^-2.  That is non-contentious, but Hansen et al proceed to ask how much of this forcing is attributable to the reduction in aerosols from shipping fuel.  They recognise that a considerable part must be due to climate feedback, that is, changes in energy absorption that are themselves due to global warming, such as a lowering of average albedo in the arctic region resulting from a reducing area of sea ice.

To address the question, Hansen et al use a spatial and temporal analysis of data on absorbed solar radiation (28).  This shows a large increase during 2020-23 in latitudes 30-60° north, which include the North Pacific and North Atlantic regions, with at most much smaller increases at other times and in other regions.  Having regard to the magnitude of this increase they infer that the forcing attributable to the reduction in aerosols from shipping fuel was of the order of 0.5 Wm^-2 (29).  This, they imply, is about six times larger than the IPCC’s formulation of aerosol forcing would imply.  It is their first piece of evidence that the IPCC’s forecasts of warming may need to be revised upwards. 

Hansen et al then proceed to consider the overall forcing due to all aerosols (which is negative because of their net cooling effect).  This is not measured, so can only be inferred indirectly.  They consider three hypotheses (or scenarios as they term them) (30).  One is based on assumptions in the IPCC’s AR6, with projection forward to 2023.  The other two, referred to as A and B, give extra weight to the non-linearity of aerosol forcing.  Each hypothesis, in conjunction with the same aerosol emission data implies a different time pattern of (negative) aerosol forcing (31).

As one test of the hypotheses, Hansen et al focus on the period 1970-2005.  They conclude that all three hypotheses are consistent with the actual global warming rate over this period, but each requires a different climate sensitivity: 3°C for the IPCC hypotheses; 4.5°C for A; and 6°C for B (32).  Those results play a key role in what follows.

Putting each of these patterns of aerosol forcing together with the known pattern of forcing from greenhouse gases and other sources such as solar variation and volcanoes, Hansen et al obtain time patterns of overall forcing for the whole period 1850-2023 (33).  Using also the above climate sensitivity results, they estimate the implied warming over that whole period for comparison with the known actual increase of 1.6°C.  It is this comparison that yields their key conclusion: the IPCC hypothesis in conjunction with climate sensitivity of 3°C is inconsistent with warming of that size, but hypothesis A or B, in conjunction with climate sensitivity of respectively 4.5°C and 6°C, can explain the increase of 1.6°C (34). 

There is further analysis in Hansen et al’s paper, but I will stop there and state what I take to be the implications of the above.  The analysis does not seem to support a clear preference for aerosol hypotheses A or B over the IPCC hypothesis.  What it does do however is support the following line of reasoning.  If the IPCC hypothesis is correct then climate sensitivity must be more than 3°C to explain the overall warming over the period 1850-2023.  If either the A or the B hypothesis is correct, then climate sensitivity must be respectively 4.5°C or 6°C to explain warming over the period 1970-2005.  In any case, therefore, climate sensitivity must be more than the IPCC’s central estimate of 3°C.  The implication is that, under any given mitigation scenario, future warming can be expected to be somewhat faster than projected in the IPCC’s AR6.

Conclusion

Most of this post is based on just three papers.  But crucially, each one takes account of new evidence that was not available to the IPCC in preparing its AR6, either because the particular studies of forests had not been undertaken, or because the unexpected warming of 2023 had not yet occurred.  I do not claim to have followed every step in their reasoning, but I find their scientific content highly plausible.  Of course, it remains possible that errors might be found, or new evidence come to light, that might undermine their conclusions.  I shall remain a sceptic, in the sense I have described.  Nevertheless, in the light of those conclusions, I have adjusted my central view – the fulcrum of my scepticism.  I now think climate change is in two respects a somewhat more serious problem than suggested by AR6: future warming is likely, other things being equal, to be somewhat faster, and the circumstances in which tree planting is an effective means of mitigation are much more limited.

Notes & References

1. IPCC AR6  Climate Change 2021: The Physical Science Basis  Chapter 7 https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/  Section 7.5.5, second paragraph below Fig 7.18.
2. NASA Charting the Exceptional, Unexpected Heat of 2023 and 2024 https://earthobservatory.nasa.gov/images/153588/charting-the-exceptional-unexpected-heat-of-2023-and-2024#:~:text=Starting%20in%20June%202023%2C%20temperatures,break%20records%20through%20August%202024.
3. IPCC AR6 Climate Change 2022: Mitigation of Climate Change  Chapter 7 https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-7/    Section 7.4.2.2, first para.
4. Hasler N, Williams C A et al. (2024) Accounting for albedo change to identify climate-positive tree cover restoration  Nature Communications  https://www.nature.com/articles/s41467-024-46577-1  Fig 1 p 3.
5. IPCC, as (3) above  Section 7.6.2.3, Box 7.10.
6. Sendzimir J, Reij C P & Magnuszewski P (2011)  Rebuilding resilience in the Sahel: Regreening in the Maradi and Zinder regions of Niger  Ecology and Society 16(3)  https://pure.iiasa.ac.at/id/eprint/9499/
7. Leudeling E & Neufeldt H (2012) Climate sequestration potential of parkland agroforestry in the Sahel  Climatic Change 115 pp 443-461  https://www.researchgate.net/publication/257547783_Carbon_sequestration_potential_of_parkland_agroforestry_in_the_Sahel
8. IPCC, as (3) above  Section 7.4.1.1, Table 3, row Forests and other ecosystems – Restore.
9. Ritchie H (2021)  Deforestation and Forest Loss  Our World in Data  https://ourworldindata.org/deforestation
10. Baggio-Compagnucci  A, Hewitt R J, et al. (2022)  Barking up the wrong tree? Can forest expansion help meet climate goals?  Environmental Science & Policy 136(3)  https://www.researchgate.net/publication/361492847_Barking_up_the_wrong_tree_Can_forest_expansion_help_meet_climate_goals
11. Baggio-Compagnucci, as (10) above, p 9, section 4.1, first para.
12. Baggio-Compagnucci, as (10) above, p 9, section 4.1, final para.
13. Baggio-Compagnucci, as (10) above, p 8, section 3.5, first para.
14. Baggio-Compagnucci, as (10) above, p 10, section 4.3, second and third paras.
15. Baggio-Compagnucci, as (10) above, p 5, section 2.4.1, second para.
16. Baggio-Compagnucci, as (10) above, p 10, section 4.3, fourth para.
17. Baggio-Compagnucci, as (10) above, p 10, section 4.3, fourth para.
18. MIT Climate Portal (2023)  Climate Sensitivity  https://climate.mit.edu/explainers/climate-sensitivity#:~:text=Climate%20sensitivity%20is%20a%20term,gases%20in%20the%20atmosphere%20doubles
19. Hausfather Z (2018)  Explainer: how scientists estimate climate sensitivity  Carbon Brief  https://www.carbonbrief.org/explainer-how-scientists-estimate-climate-sensitivity/
20. Myhre G, Myrhe C E L et al. (2013)  Aerosols and their relation to global climate and climate sensitivity  Nature Education Knowledge 4(5): 7  https://www.nature.com/scitable/knowledge/library/aerosols-and-their-relation-to-global-climate-102215345/
21. NASA CERES  https://ceres.larc.nasa.gov/#:~:text=The%20CERES%20instruments%20provide%20direct,the%20ultraviolet%20and%20far%2Dinfrared.
22. Myhre, as (20) above,  Box Figure 4.
23. Hansen J E, Kharecha P et al (2025)  Global warming has accelerated: are the United Nations and the public well-informed?  Environment 67(1)  https://doi.org/10.1080/00139157.2025.2434494  p 7
24. International Maritime Organization (2021)  IMO2020 fuel oil sulphur limit – cleaner air, healthier planet  https://www.imo.org/en/MediaCentre/PressBriefings/pages/02-IMO-2020.aspx
25. Jia H & Quaas J (2023)  Nonlinearity of the cloud response postpones climate penalty of mitigating air pollution in polluted regions  Nature Climate Change 13  https://www.nature.com/articles/s41558-023-01775-5
26. Hansen, as (23) above, p 14, first para.
27. Wikipedia – Solar Irradiance – At the top of Earth’s atmosphere  https://en.wikipedia.org/wiki/Solar_irradiance#At_the_top_of_Earth’s_atmosphere
28. Hansen, as (23) above, p 16, Fig 9.
29. Hansen, as (23) above, p 15, second para.
30. Hansen, as (23) above, p 17, last para. continuing p 18.
31. Hansen, as (23) above, p 18, Fig 13.
32. Hansen, as (23) above, p 21, last para. continuing p 22.
33. Hansen, as (23) above, p 21, Figs 16a/b.
34. Hansen, as (23) above, p 22, second para.