After a perceived ‘temperature plateau‘ of about a decade, global temperatures seem to be rising faster than ever before. First 2014 broke the global record for hottest year (then held by 2010). Then 2015 broke that record. And 2016 in turn is set to completely smash that record. We’ve just had the 15th hottest month in a row globally, and counting.

So, what’s going on? We decided to properly investigate – and wrote a 25-piece series about (what we call) the ‘Real’ Global Temperature Trend. Central question: Are these ‘insane’ peaks on the global temperature graph really peaks? Or is half the story hidden still – hidden in masking factors, hidden in oceans, hidden in climate science…?

Today we finish the series and present our 5 main conclusions here at Bitsofscience.org – introducing a new quantity (RGT), supplemented with a hopefully very illustrative graph, and a summary of the entire series below:

What we conclude about the global temperature trend: 

1. A large part of atmospheric warming is still masked by (shorter-lived) cooling factors and by climate system inertia – therefore the CO2-coupled ‘Real’ Global Temperature is (much) higher than currently observed temperatures.
   
2. Recent global temperature records were no ‘peaks’, but rather corrections to a climatic temperature trend line that is (much) higher than the statistical trend line.
   
3. If atmospheric CO2 is stabilised around the current level (404 ppm) there is an uncertain, but possibly large amount of ‘pipeline warming’. This warming in the pipeline may lead to an additional temperature rise of more than 1 degree Celsius – additional warming that will manifest itself after stabilisation of the CO2 concentration. The final temperature rise of the current CO2 concentration could be up to 2 or 3 times as high as the warming that is currently observed(!)
   
4. The current atmospheric CO2 level is a dangerous overshoot – to stay below internationally agreed climate targets (both 1.5 & 2 degrees) the CO2 concentration (that is currently still rising year by year) should not be stabilised, but should in fact be lowered.
   
5. If we keep measuring climate change by the observed rise in live temperatures and the Earth & climate system responses this temperature rise causes (including extreme weather events) we keep underestimating the real scientific climate urgency.

Graph shows (bottom) rising atmospheric CO2 concentration and (top) observed global temperatures (NASA GISS), plus three different values for ‘Real’ Global Temperature (based respectively on climate sensitivity, ocean thermal inertia and – additional expected warming at the CO2 level of that respective year. All temperatures relative to ‘late pre-industrial’ climate baseline. Graph made by climate data journalist Stephan Okhuijsen (Datagraver.com) for the ‘Real’ Global Temperature series of Bitsofscience.org. For full resolution image go to our special graphs page.

[DISCLAIMER: to create above graph we had to make several assumptions, which are explained throughout this article. One thing that deserves special attention is that we draw climate sensitivity as a direct line between preindustrial CO2 (280ppm) and doubled CO2. It’s a nice linear line, but an oversimplification and not correct. In reality warming increases logarithmically with CO2 – the line starts and ends at the same spot, but ought to be more convex shaped in between. The difference is quite large. We use as formula [Tcs = Tpre-indus + ((CO2 – 280)/280) x 3] to get to +1.39 Celsius in 2016. It should be according to [forcing = 5.35*ln(CO2/CO2_ref)​] – therefore at 400ppm you already get 51% of warming, so +1.54 degrees. This means the line for Tclimatesens should be higher than graphed above.]

How do recent heat records compare to the global temperature trend?

2014, 2015 and 2016 subsequently broke all the world’s temperature records. That’s why at Bitsofscience.org we started a 25-part series investigating the climate science behind the warming – with one important question: are these ‘insane temperature records’ really peaks on the trend line (as observations suggest), or are we somehow underestimating the warming potency of CO2 and are these temperature ‘peaks’ mere corrections to a higher ‘real’ global temperature trend…?

Today, after quite a lot of hard work, we present the conclusions. We would like to thank the many climate scientists who kindly and freely helped us with this investigation – and the many more whose invaluable (ongoing) research we could draw from!

Introduction to the ‘Real Global Temperature’ (RGT) – and 4 different global temperature trends

This is the final of the ‘Real Global Temperature Trend’ series at Bitsofscience.org – a scientific investigation of observed global temperatures, masking factors (global dimming, aerosol reflectivity), ocean thermal inertia, climate sensitivity, ‘Earth system sensitivity’ and paleoclimatology. In this conclusion we suggest a new quantity, the ‘Real Global Temperature’ (RGT): the (future) temperature coupled with the (current) atmospheric CO2 concentration.

‘Real’ Global Temperature (RGT) = (future) global temperature linked to a specific CO2 concentration

We try to show that this temperature value is likely substantially higher than currently observed global temperatures, by defining four different temperature trends: 1) the statistical trend of all observed temperatures, 2) a CO2-coupled temperature trend line based on climate sensitivity estimates, 3) a temperature trend line that includes ocean thermal inertia, 4) a long term ‘Earth System Sensitivity’ CO2-coupled temperature trend line, deduced from paleoclimate comparisons.

What have we done to create key temperature trend insights?

1. First we’ve closely followed observed temperatures from established global temperature datasets. This enables to understand and to draw a clear (statistical) trend line – one that is higher than La Niña-dominant years (2011-2013), comes closer to ENSO-neutral years (2014) and is lower than El Niño-dominant years (2016). The climate average is derived using (centred) 30 year average values. This trend line we call observed global temperatures. (We’ve used extrapolation of last possible decade of centred 30-year climate average to continue this statistical trend line to 2016.)
   
2. Secondly we’ve looked at various quick-acting masking factors – factors like global dimming and aerosol reflectivity. Because these cooling factors are (compared to CO2) relatively short-lived – they create a warming time lag. Some authors go as far as to state up to 50 percent of the CO2-induced warming is masked by global dimming – suggesting ‘real’ temperatures are twice as high as the currently observed temperatures, by this one factor alone.
   
3. Thirdly we’ve looked at slower acting climate system responses, a lot of climate sensitivity studies, a climate senstivity experts survey (16 leading experts contributed!), ocean thermal climate inertia (& ocean carbon cycle climate inertia) and ‘Earth System Sensitivity’ – deduced from paleoclimate comparisons.

Explaining the difference between a statistical trend, and a ‘real’ (climatological) trend line

Much of the warming of the current elevated atmospheric CO2 level is still being masked by short-lived cooling factors (aerosols), and much of the eventual warming of this CO2 level is hidden in inert climate and Earth system processes. This leads to grave underestimation of the possible long-term climate response – with a final temperature rise that could be 2 or 3 times as high as the currently (30-year climate average statistical trend line) measured warming.

If the atmospheric CO2 concentration would be stabilised around the current level, the world is set to experience a substantial amount of ‘pipeline warming’ – delayed additional warming, increasing with time scale: some 0.3 degrees Celsius based on (median/consensus – see our expert survey) climate sensitivity (mostly fast-acting atmospheric and sea-ice feedbacks), possibly 0.7 degrees based on ‘ocean thermal climate inertia’ (at a multiple decades time scale (25-50 years)), and possibly slowly increasing to 1 to 2 degrees additional warming to combined ‘Earth system sensitivity’ processes (ice sheet melting, ocean current decline, vegetation changes, Earth albedo decreases – still excluding (by definition of stabilised CO2!) many possible large-scale carbon feedbacks) – deducing from ‘relatively recent’ paleoclimate (Pliocene & Eocene) comparisons.

Uncertainties grow larger with time. Climate Sensitivity is useful for near future, paleoclimate comparisons show possible (long term) effects

These different ways to calculate and define the ‘Real’ (future) Global Temperature at any given CO2 level shows large remaining scientific uncertainties – uncertainties that increase with time scale (one of the main reasons why (the possibly conservative) metric of ‘Equilibrium Climate Sensitivity’ is used/preferred in political context – climate sensitivity offers relatively high certainty for near-future warming projections). The mostly paleoclimate-deduced (and less well defined) ‘Earth System Sensitivity’ show the possible maximum extent of CO2’s long-term climatic effects.

Main conclusion: recent temperature records are no peaks, but corrections to trend

The three different RGT trend lines (climate sensitivity, ocean inertia & paleoclimate) show large remaining uncertainty about the final magnitude of climatic warming for a given concentration of CO2. They all have one thing in common though: they draw a higher (climatological) global temperature trend line, than the (statistical) centred 30-year average temperature trend.

Comparing climate records to trend lines:

Global temperature records of 2014 and 2015 are no peaks on the statistical trend line. The record heat of 2016 (influenced by El Niño & Arctic feedbacks) is a small peak on this trend – as the graph shows.

Compared to the three different climatological (RGT) trend lines all (annual – see thin black line in graph) recently observed temperature records lie under the real trend line. In that sense these recent ‘hottest years’ records are no peaks, but ‘corrections towards the trend’. (Also visible in the graph is that individual monthly global temperature records can be peaks on one of the RGT trend lines, namelijk the consensus climate sensitivity line. This goes for instance for the monthly global temperature record of February 2016. All monthly records are still well below ocean inertia & paleoclimate RGT trend lines.)

Summary of the series:

Part 1 – Statistical Likelihood: Nature study shows a “99.9999%” chance that observed temperatures show a rising trend.
Good to know before you start off: yes, science says it’s not just noise, there is an actual trend. That is still a chance of 1 in a million that people denying that the world is warming are right. (That chance has grown slimmer still with the recent set of global temperature records of 2014, 2015 and 2016 – deviations that further lift the statistical temperature trend line.) Of course observed temperatures are always influenced by natural climate variations too, most importantly the alteration of La Niña and El Niño-dominant years. (The 2016 global temperature record is provoked by El Niño and other factors, like Arctic feedbacks.)

Part 2 – Global Dimming: if aerosol cooling is underestimated, then warming trend is higher.
Very important: through energy consumption and land use we do not just emit greenhouse gases, we also emit aerosols – with reflecting, absorbing and blocking properties. Aerosols have a shorter halflife than CO2 – therefore: if the net cooling potential of aerosols is underestimated – the same might go for the warming potential of CO2 – in order to explain the observed temperature dataset.

Part 3 – Global Dimming 2: although global dimming is declining, it could still mask 50% of CO2 warming
The different climatic effects of aerosols are dominant at different atmospheric altitudes. On of the first studies to take this into account was published in Nature and indicates that indeed aerosol cooling might be underestimated – deducing CO2’s warming effect is about 50 percent higher (around the upper range of IPCC climate sensitivity).

Part 4 – ‘The Lukewarmers’: if aerosol cooling is overestimated, then the opposite holds true: CO2 warming would be smaller
Aerosol climate forcing remains an important factor with a relatively large uncertainty range. We looked at a study that proclaimed an opposite effect: low aerosol cooling, therefore lower than thought CO2 climate sensitivity. (Sadly it’s quite illustrative of lukewarmer publications – the creation of low-end climate sensitivity outcomes, using cherry picked data [this case: skipping volcanic periods] and biased assumptions [low aerosol forcing]. Bad Science.)

Part 5 – Climate Sensitivity is higher when models take subtropical cloud-decrease feedbacks into account
The real-world climate system is very complicated, therefore modelling it, is an endless refining act. Various possible cloud feedbacks still contribute to CO2 climate sensitivity uncertainty – as different clouds have different climatic effects. You don’t want to hear climate change increases warming-type clouds. You don’t want to hear climate change decreases cooling-type clouds. Here we show it may do both, increasing net climate sensitivity outcomes (in the featured study by decreasing natural climate cooling over the subtropics).

Part 6 – Observations: THIS is the line you get when you combine ALL established temperature measurements
It sounds easy and it sounds good, combining and comparing all global temperature measurements of five well-established temperature datasets – but of course reality is never as easy. Here Stephan Okhuijsen integrated the temperature measurements of NOAA NCDC, NASA GISS, RSS (remote sensing systems), Met Office Hadcrut4 and JMA. Unfortunately they all use different baselines – and none uses the politically important pre-industrial values as base [as we do ourselves in the graph at the top of this article]. So this is an indication only. Quick conclusion: everyone agrees temperatures are going up – most agree on individual extremes too.

Part 7 – Volcanoes & Climate Sensitivity: ‘Volcanoes did not cool the Earth in the 1980s, so climate sensitivity is at least 2.1 to 8.9 degrees Celsius’
When you want to assess the sensitivity of the climate system you have to take special time to look at volcanoes. These publications offer an interesting line of thinking: a lack of observed global cooling in periods of increased tropical volcanic activity (the 1980s and 1990s) – also a period in which the increase in atmospheric CO2 concentrations accelerated, may indicate high to very high climate sensitivity.

Part 8 – Volcanoes & Climate Sensitivity: Major eruption of Mexican Popocatépetl volcano can cause temporary cooling
This extra article we added to show that despite the certainty that global climatic warming will continue over both the near- and long-term future, scenarios are imaginable in which temporary cooling phases let temperature measurements dip well below the (rising) trend line. And even though the cooling potential of volcanoes may sometimes be exaggerated it is good to note that not all volcanoes are equal. One very sulphur-rich explosive volcano that could potentially cause a dip in global temperatures is currently active: Popocatépetl – in Mexico. It will not change the actual global temperature trend though.

Part 9 – Climate Sensitivity: ‘Not all climate forcers are equal, so climate sensitivity is higher,’ says NASA
“Modelling the complexity of system Earth is difficult, but trying harder helps.” Again, modelling climate change is an endless refining act. We’ve seen previously in our temperature trend series how you need to take into account the 3D effects of aerosols, we’ve looked at different cloud feedbacks. And now the latest research by one of the best climate research institutes shows we also still need to better take into account factors like geography, for instance the difference between a land mass-dominated northern hemisphere, and ocean-dominated southern hemisphere. Incorporating such insights NASA researchers conclude ozone, volcanic and land use climate forcers have been misrepresented in models and (therefore) climate sensitivity is likely not in the low end of the established (IPCC) climate sensitivity range.

Part 10 – Climate Sensitivity: Refining cloud feedbacks lifts climate sensitivity to 5-5.3 degrees, say Yale researchers
A 2016 bombshell climate study, published in Science. “Water seems easy to understand, but is in fact a very complicated climate factor.” We’ve learned before that various clouds have very different climate interactions. Refining understanding of one important climate-cloud feedback – ice formation in sub zero cloud tops – leads researchers of Yale and the Lawrence Livermore National Laboratory to conclude climate models overestimate cloud reflectivity in warming scenarios. These warming scenarios should therefore be revised upwards, they say. Complicated story, therefore a long article.

Part 11 – Emissions Budget: Under agreed 1.5 degrees target world has 8 years of (current) emissions left for rest of century
We thought it would be useful, while we are investigated the global temperature trend, to assess how this trend relates to another line, one we all promised not to cross: the new UN climate target from the Paris climate summit of December 2015, keeping warming ‘well below’ 2 degrees, aiming for a maximum of 1.5 degrees above pre-industrial climate. Warning: that 8 years emissions budget is only if you ignore the pessimistic science – see the latter part of the series (about long-term climate inertia) – the 1.5 line may very well have already been breached, indeed, by the ‘real’ global temperature trend.

Part 12 – Observations: March 2016 breaks February records, says JMA. So: have +1C peaks become the new monthly norm?
Important aim of our series is to see how currently observed global temperatures relate to the global temperature trend line. But then of course we need to also spend time to properly measure just how high these ‘peaks’ really. March 2016 was the hottest month ever measured, according to the Japan Meteorological Agency, beating February. Both months were over 1 degree Celsius warmer than the climate average value between 1900-1999 (yet another baseline!) – closer to 1.5 degrees above pre-industrial climate.

Part 13 – Observations: First 3 months 2016 are top 3 hottest months, 2016 now ‘certain’ to become hottest year on record
More observations, more temperature records. This time a different temperature dataset – NASA GISS. The first three months of the year are the top 3 months with the highest monthly temperature deviations ever measured. That’s remarkable.

Part 14 – Observations: World has 12 monthly records in a row – global temperature jumps 0.2 degrees Celsius in one year
We weren’t quite there when we wrote this article, but now we can confirm: from May 2015 to April 2016 the world has experienced 12 subsequent global monthly temperature records (so each single month broke the existing record in the historic record for that month). That’s really remarkable. As a consequence global temperatures jumped by 0.2 degrees Celsius. (Extropolate that temperature rise and you have a warming trend of 2 degrees each decade – just to illustrate the steepness of recent climate events.)

Part 15 – Trend Forecast: 2016-2010 5-year global average temperatures forecast to be above 2015 temperature record
The global average temperatures between 2016 and 2020 will be higher than the ‘peak’ of the global temperature record of 2015 – despite an also forecast La Niña episode (which leads to upwelling of cool deep ocean waters, therefore a dip in atmospheric temperatures). That’s what we learn from an interesting temperature trend analysis, performed by the UK Met Office’s Hadley Centre. It’s very interesting, because it’s a first step investigating whether recent (annual) global temperature records were in fact peaks on the trend line – or mere dots on that line…

Part 16 – Climate System Thermal Inertia: trend line is 0.6 degrees Celsius higher than observed temperatures show
Sometimes you would wish all the world would read one single climate study. Not for the exact figures & conclusions, but for the important specific subject it helps to explain. That goes for this study, performed by 15 climate scientists led by (then head of NASA GISS) James Hansen, and published in Science in 2005. It’s probably the most frequently quoted study about the warming lag of Earth’s climate system: depending on the heights of the CO2 concentration a certain amount of heat is -constantly!- being created in the atmosphere. A large chunk of this heat is absorbed by the oceans, that in turn also -slowly- warm up. Once the atmospheric CO2 concentration is kept constant, this atmosphere-ocean heat absorption slowly decreased, and therefore atmospheric temperatures rise further. This influential study estimates Earth’s energy imbalance (at 2005 CO2 level! → ~378ppm) to lie around 18 w/m2, and therefore this ‘ocean thermal climate inertia’ (again, in 2005) to lie close to 0.6 degrees Celsius of additional warming – manifesting over a warming time lag between 25 to 50 years. (As the CO2 concentration continued to rise (even at increasing speed) between 2005 and 2016 it is likely this ocean thermal climate inertia is now higher than in 2005 – with not 0.6, but closer to 0.75C ‘pipeline warming’.)

Part 17 – Climate System Thermal Inertia: it is lower when you don’t assume a CO2 flatline
Part 17 of our ‘Real’ Global Temperature Series serves to place part 16 in proper perspective. Ocean thermal climate inertia is often misquoted as ‘inevitable warming once anthropogenic CO2 emissions drop to zero’. Fortunately that is not true – because the Earth’s climate system is more complicated still. Apart from absorbing heat the oceans also absorb a big chunk of the CO2 we emit to the atmosphere (currently about a third – but decreasing with ocean warming!). This ocean CO2 absorption is not good as it creates another unwanted effect: ocean acidification. But it also alleviates some of the climate warming we would otherwise have. And when we stop emitting CO2 to the atmosphere also this absorption continues for some time (‘ocean CO2 climate inertia’) – more or less precisely compensating (sheer coincidence) the ocean thermal climate inertia. [Therefore the latter should be interpreted as ‘expected (inevitable) additional warming, once atmospheric CO2 concentrations are stabilised – a scenario of very drastic, but not 100% emissions reductions.]

Part 18 – Climate Sensitivity: now how high is it really? Here’s the answer of the world’s 16 leading climate experts!
By far the most fun and we think most interesting to read part of the series. We’ve asked the world’s 16 leading experts on the subject of climate sensitivity about their ‘scientific gut feelings’ – “if you would have to pick one number (degrees Celsius) for Equilibrium Climate Sensitivity (ECS) – what would be your best guess?” For us creating this article was probably the most instructive part of the series, because on the one hand it’s such a scientifically complicated subject, with such a challenging pile of research to read through to better understand the margin of uncertainty – and on the other hand it’s clearly politically disputed (with BAU forces having a lot to gain from lowered estimates!). Spoiler alert(!) – what we learned from the experts: almost all agree on a likely value close to the centre of the IPCC uncertainty range, so a climate sensitivity value around 3 degrees Celsius – and a clear majority of experts said it’s sooner (a bit) higher than 3C than lower. Different experts also make a distinction between (quick-acting) climate sensitivity and other metrics for expected warming (and other climatic changes), for instance ‘Earth system sensitivity’ – slower acting, and of larger magnitude. (We spent the final part of the series to better understand this Earth system sensitivity!)

Part 19 – Observations: There is hot, hotter, hottest – and 2016: dangerously close to pre-industrial +1.5 degrees…
In 2016 you just have to return to live temperature observations every now and then. Here we show the dramatic extent of the unfolding 2016 global temperature record, as compared to other recent ‘hottest years’: in the first months of 2016 we almost crossed the pre-industrial +1.5 degrees Celsius line, the line just a couple of months ago, during the COP21 UN climate conference in Paris agreed we should ‘try’ not to cross…

Part 20 – Climate Inertia: Combining ‘thermal inertia’ & ‘carbon cycle inertia’: still one decade extra warming after emissions stop
To complement earlier articles (part 16 & 17) about climate inertia here is a piece that takes a closer look at the combined effect of thermal climate inertia and carbon cycle climate inertia. Conclusion: warming may continue for about one decade after anthropogenic CO2 emissions drop to zero. Based on a 2014 publication in Science by Katharine Ricke & Ken Caldeira.

Part 21 – Emissions Budget: “Science is ruthless” – we have ‘about zero’ years of emissions to stay below the 1.5 degrees climate target
Update of part 11, about the remaining atmospheric emissions budget – adjusted for understanding of both thermal climate inertia and carbon cycle inertia. Sobering conclusion: breathing space to stay below the internationally agreed 1.5 degrees target may be fully blown. (That means early 2016 global temperature records – see for instance part 19 – when observed global temperatures closed in on ‘pre-industrial +1.5’, may be quite indicative of where the ‘real’ global temperature trend lies.)

Part 22 – Emissions Trajectory: World still on course for dramatic 3 degrees warming, need to more than double reductions speed
Here it may become confusing, but we had to do a reality check. In the temperature trend series thus far we discuss additional warming after the CO2 concentration is stabilised. But reality of course is worse, as the CO2 concentration is still rising. And even if we combine all the different conditional and unconditional emissions reduction targets of individual nations after the important COP21 UN climate summit in Paris, in December 2015 – this is set to continue. (It’s good to note that this ~3 degrees warming trajectory is in line with the ‘consensus climate sensitivity’ RGT trend line – but that it could still be an underestimation judging by the ‘ocean inertia’ and paleoclimate RGT trends.)

Part 23 – Climate Sensitivity has large geographical spread: 2-3 degrees equals 6+ degrees warming in the Arctic
In order to be able to assess climate impacts it’s very important to understand climate change manifests itself in different ways across the globe, with large regional differences. In part 23 we focus mostly on ‘Arctic amplification’ – also because we need that understanding to be able to better draw lessons from the paleorecord later on. Good to know: a global average warming of 2-3 degrees leads to more than 6 degrees warming in the Arctic.

Part 24 – Earth System Sensitivity: Paleoclimate tells we may have 3 degrees additional warming in pipeline at current CO2 concentration!
If ever you want to accuse us of being alarmists, here’s your chance! This is beyond anything in terms of CO2’s long-term warming potency: even at the current CO2 concentration we may have 3 degrees additional warming – that’s warming on top of observed temperature rise(!) This we conclude from mid-Pliocene Arctic temperatures (and Eocene comparisons) at historic CO2 concentrations close to 400 ppm – and calculating back to global average temperatures using reversed Arctic amplification. We omitted this extreme value from the graph and our main conclusions – and decides to further investigate the issue in the last part of the series.

Part 25 – Earth System Sensitivity: Paleoclimate experts to the rescue! Closer to 1-2 degrees Celsius in pipeline from long-term climate inertia
“Once you conclude there could be 3 degrees in the pipeline – you either want to move to one of Jupiter’s moons, or you want to be wrong.” For the last part of the temperature trend series it was time to call in some more help from the real (paleo)climate experts: where’s the flaw in part 24. Well, perhaps in that Arctic amplification value – as Arctic amplification ratios may actually increase with warming. And there is high uncertainty everywhere, starting with the paleo CO2 record. All in all from the Pliocene record we can still deduce the possibility of large additional warming at the current atmospheric CO2 concentration. Also a nice chance to better look at wat ‘Earth System Sensitivity’ actually is, and where & why it differs from for instance Equilibrium Climate Sensitivity.

© Rolf Schuttenhelm | www.bitsofscience.org