Speaking of which, there's a new paper out declaring that the  anthropogenic influence in the atmosphere has been detected with 5 sigma confidence.  I'm not sure how much sense such statistical measures make in climate science. 

In any field of science, statistical measures of confidence in a model or hypothesis works by comparing observations to the predictions of that model vs. the predictions from a null hypothesis.

In this case, the standard method for testing the attribution of warming to anthropogenic causes is to run two types of climate models: one set which includes both natural and anthropogenic changes in Earth's energy balance (the radiative forcings), and the other set including only the natural forcings.  Then the models run through the physics and climate dynamics to predict the change in Earth's temperature over time under those scenarios.  Finally, compare those predictions to what we actually observe.  What we observe is consistent with models including both natural and anthropogenic forcings, and strongly inconsistent with natural forcings alone.  

We might wonder if internal climate dynamics throws a wrench in these conclusions.  How can we be sure that the observed warming is not due to some other cause besides the anthropogenic forcings?  There are several sources of confidence:

• The additional thermal energy from global warming can be traced through the whole system (most of it is being absorbed in the oceans).  The signal of global warming in surface temperature alone can be tricky, because the atmosphere stores only a small fraction of the total energy and much energy is transferred between atmosphere oceans and cryosphere, but when we check the whole system we find it agrees with the change in Earth's energy balance and how the energy is transferred between subsystems.
• We can be confident that the models are mostly capturing the physics correctly, because they do a good job of retrodicting past climate changes caused by solar variability and volcanoes.  When run backwards, they don't predict nonsense.
• The enhanced greenhouse effect is measurable by its altitude dependence.  It warms the surface and lower atmosphere, but cools the upper atmosphere, and observations reflect this trend.

The much more uncertain thing in climate science at this time is how much the average surface temperature warms under certain amount of greenhouse gas emission, which leads to the subject of climate sensitivity.  Understanding of this gets better over time, but still has a pretty wide spread of possibilities, from about 1.5C to 4.5C for a doubling of CO2 concentration.  We can be very confident that it is not less than 1C and not more than 6C.

Plus there is also the factor of speed. It is an old factoid, but bears repeating: no-where in Earth's history has the climate warmed in such a short timespan so expediantly. Outside of really obvious forces like super-volcanic eruptions and asteroidal impacts, the modern extent of changes in greenhouse gas levels and subsequent rise in global temperature hasn't been found in fossil records. Yes the planet has cooled and heated up dramatically in the past (even moreso then now), but only over some thousands or millions of years. 

The much more uncertain thing in climate science at this time is how much the average surface temperature warms under certain amount of greenhouse gas emission, which leads to the subject of climate sensitivity.  Understanding of this gets better over time, but still has a pretty wide spread of possibilities, from about 1.5C to 4.5C for a doubling of CO2 concentration.

And that wide range has remained pretty much the same for decades despite much research.  Headlines like 5 sigma, 97% agree, etc, are somewhat misleading.  The press gladly interprets "97% of scientists agree that if you roll a dice, the result will be less than 7" as "scientists can predict the roll of a dice with stunning confidence".  But worse, climate hypochondria is everywhere.  We don't know well what the climate of the future will be, but let's say that CO₂ will level out at 750 ppm and there will be 3.5 degrees of warming.  Something like that could be the least controversial projection.  We already have 1 degree of warming and will have 2.5 degrees more in store.  But where's the 5 sigma or 97% agree research proving that this is very, very bad in this and that very specific way?  Like hypochondriacs will search the literature for symptoms that could possibly describe what they feel and see, the new climate hypochondriacs do likewise.  "Could" becomes "will", and necessity and sufficiency become the same.  Since we're 1 degree on the way, we should see clear signs of what is to come.  People sure do - by attributing anything bad, also declaring any change bad, to climate change.  I wonder how future generations will describe the craze.

 Yes the planet has cooled and heated up dramatically in the past (even moreso then now), but only over some thousands or millions of years. 

During the transitions between ice ages, temperatures have changed quite dramatically over decades, but it's hard to say exactly how fast as such proxy records have low temporal resolution.  In ice there is a mixing of gases for decades.  Search for GISP2 and you'll see dramatic changes in the past over short time.  The current warming is hardly visible, but mainly because of the mixing time.

In ice there is a mixing of gases for decades.  Search for GISP2 and you'll see dramatic changes in the past over short time.  The current warming is hardly visible, but mainly because of the mixing time.

To be honest, this is a rather weak argument. In all fairness, it does validate the notion that the Earth has fluctuated dramatically in the past without our help. But still doesn't dilute the idea that humans have had a greater impact on our climate (or better or worse) then nature.

The planet had been leaning towards a global cooling since the late Miocene epoch. This trend was caused by various factors including continental drift and interaction, desertification and ice-cap formation. Eventually, during the end of the Pliocene epoch, this global change broke through and turned the world into a more temperate place, ending in the Pleistocene epoch that we all know and love, with its ice ages and whatnot. The melting and refreezing of icy glaciers did indeed release a lot of CO₂and CH₄,



-- but these are clearly reflectant of relatively natural processes and did not have a prolonged presence in the atmosphere, nor demonstrated an expedient rise in resultant temperature (relative to other effects):



That global temperature and weather has changed in drastic ways is not at question here - rather it is that we cannot use nature's past work to conveniently handwave a lot of our measurements as artifacts of experimental limitation or anthropocentric industrial elitism (to say that we think we caused it because we are so special and have dominion over the planet).

Care must be taken to use the appropriate dataset for the intended purpose.  If you're looking at ice core data, it will emphasize regional climate changes as much as global ones.  For example, fluctuations at the end of the ice ages tend to be regional, and the largest are due to changes in ocean circulation, as lakes of fresh water dammed behind ice suddenly break free and enter the ocean.  

Global temperature reconstructions are made from a variety of proxies all across the world, of which ice cores are a subset.  Globally, the most rapid temperature changes are caused by volcanic eruptions, with the planet being cooled by a few degrees Celsius for a few years due to the blanket of aerosols.  A large asteroid impact will also do this, though we don't have as good resolution data of this effect in the geologic record.

Otherwise, in the direction of warming temperatures, the closest analogue to what we are experiencing today is the pulse of warmth during the Paleocene Eocene Thermal Maximum, which raised global temperatures by about 5C (and much more in the arctic), lasted about 100,000 years, and was caused by a release of carbon (probably originally as methane from hydrates) into the atmosphere.  It's an excellent case study for global warming.  But even that warming episode was about 10 times slower than what we are doing now.  Our rate of emissions is faster.

To be honest, this is a rather weak argument. In all fairness, it does validate the notion that the Earth has fluctuated dramatically in the past without our help. But still doesn't dilute the idea that humans have had a greater impact on our climate (or better or worse) then nature.

I was addressing sensational claims about climate not having changed as fast as in the last century. It's most likely far from the truth.  The cause itself for climate change does not affect whether the change could lead to some biological catastrophe.

For example, fluctuations at the end of the ice ages tend to be regional

Absolutely, and the regional changes are what matters most.  That's what animals have to adapt to.  If polar bears experiences that the sea ice melts away, they don't care if the tropics cooled and the global average were the same.

This is relevant because there are so many claims that we're experiencing unprecedented change driving species to extinction.

Regional climatic shifts at rates of 2C/decade, like on Svalbard between 1995-2015, are by no means uncommon.  The temperature rose by 5C between 1915 and 1935 (before more slowly cooling again).  Species migrate rather than going extinct.  For instance, blue mussels may have returned to Svalbard after a 900 year absence (if the climate keeps staying mild).  (Blue mussels can be found in great numbers across Svalbard, but usually several thousand year old shells only)

Otherwise, in the direction of warming temperatures, the closest analogue to what we are experiencing today is the pulse of warmth during the Paleocene Eocene Thermal Maximum, which raised global temperatures by about 5C (and much more in the arctic)

But few claim that the current warming will reach 5 degrees or more, and this event happened in an era that was already very warm.

no-where in Earth's history has the climate warmed in such a short timespan so expediantly. Outside of really obvious forces like super-volcanic eruptions and asteroidal impacts, the modern extent of changes in greenhouse gas levels and subsequent rise in global temperature hasn't been found in fossil records.

Stellarator is being very clear that he is referring to global warming, not regional climate change which of course happens more or less constantly.  His claim that the rates of increasing radiative forcing and associated rise in global temperature is unprecedented in known Earth history -- is correct.

This is relevant because there are so many claims that we're experiencing unprecedented change driving species to extinction.

The opinion spectrum is very wide and I think it is not hard to find claims where on one end the whole biosphere is doomed by global warming, or on the other end it its impacts are negligible compared to other factors.  The consensus of scientific opinion is somewhere in the middle.  Some species are stressed by the changes associated with global warming more than others, and some species can migrate more easily than others.  Coral reefs, for example, are already experiencing higher mass bleaching rates and are at high risk of vanishing altogether under more than about 2 degrees of global temperature rise.  The Amazon is highly at risk for global temperature rise in excess of about 4 degrees, even if we halted the deforestation and preserved those lands.  Both of these ecosystems represent a high amount of biodiversity.

There are also many examples of species that are threatened by increasing global temperature because they cannot adapt or migrate quickly enough, or are otherwise in niche environments.  Sometimes the loss of these species is used to promote claims that we are currently in a mass extinction, though the problem there is that these species are unlikely to be represented in the fossil record, so this is an over-estimate of species loss compared to mass extinctions in the fossil record.

Another aspect of warming-induced migration occurs with species we don't want to spread.  With globally rising temperatures there is a spreading of pests and infectious diseases.

But few claim that the current warming will reach 5 degrees or more, and this event happened in an era that was already very warm.

True, but I think this misses the point of the example.  The PETM had the fastest rise in global temperature known in the geologic record, and yet was still slower than what we're doing now.  It puts in perspective just how large our emissions are and how rapidly the Earth is changing because of it, compared to natural events.

But even more importantly, the PETM demonstrates that a similar magnitude of greenhouse gas emission did warm the planet, and that the associated effects that we are expecting from our current greenhouse gas emissions also came with it.  Sea level rose several meters due to thermal expansion, precipitation patterns changed, and the arctic became subtropical.  Half of all foraminifera (ocean dwellers that build carbonate shells) went extinct due to rising temperature and ocean acidification, with effects that worked up the food chain.  Plants showed the CO2 fertilization effect, but became less nourishing, and increased damage from pests is also evident. Today we have a similar concern about decreasing agricultural productivity at low and mid latitudes.

The lesson of the PETM is that the predictions of the IPCC synthesis reports are substantiated by what actually happened.  Is it a threat to our existence or all life on Earth?  No, and the geologic record proves it.  But it is a pretty serious issue, and we face a wide variety of challenges because of it.

His claim that the rates of increasing radiative forcing and associated rise in global temperature is unprecedented in known Earth history -- is correct.

That depends on the associative rules of English, e.g. whether "A and B is unprecedented" means (A and B) is unprecedented or "A is unprecedented and B is unprecedented".  I don't know if the singular verb was intentional and the former is the meaning.  I don't think we have data to claim that A (change in greenhouse gases) is unprecedented, we just don't know exactly how what mechanism it would take, and we don't have data with sufficiently fine time resolution to know that B (decadal temperature rise) is unprecedented, but for that we have many hints that it isn't (during and near glacial transitions, volcanos, asteroid impact).  But I was clear that I was addressing climate change.

There are also many examples of species that are threatened by increasing global temperature because they cannot adapt or migrate quickly enough, or are otherwise in niche environments.

It's in the nature of niches means that there are threats. Such vulnerabilty is nothing new.  Change, to a certain degree, can also be a driver for diversification, as they also open up new possibilities.   But I'm not saying that climate change is not a real threat to some species, but at a global level the threat has been misunderstood.  Humans have totally changed the land of half of the Earth, and actively shifted the animal populations through domestication.  Does climate change receive attention that is proportional to that degree of impact.  Even proportional within an order of magnitude?  If we reanimate Darwin and send him on a new Beagle expedition to reassess the biodiversity 200 years after, would he really relate the changes to temperature rise?

But it is a pretty serious issue, and we face a wide variety of challenges because of it.

Sure, it can be framed that way, but does Earth benefit from all the attention it gets?  Climate change is portrayed, with much help of the scientific community, as the main environmental challenge of today.  That, I think, is a serous issue.  Because, frankly, it isn't the main challenge.  If the wild animals of the world could vote between a world at 1850 temperature with our 8 billion presence and a 3 degree warmer world, gradually increased over 250 years, without us, what would it be?

 I don't think we have data to claim that A (change in greenhouse gases) is unprecedented, we just don't know exactly how what mechanism it would take, and we don't have data with sufficiently fine time resolution to know that B (decadal temperature rise) is unprecedented,

I wasn't going to get any further involved in this conversation, but this piqued my interest. We actually do know when CO₂ levels rose and how far in the past. By making some basic assumptions on how greenhouse gasses effect temperature, we can say that A is unprecedented, either in a regional or global sense. If we cannot directly measure these fluctuations, then we can see it's effects on the animals that were fossilized at that time, or in certain sedimentary deposits. From these studies, we know CO₂ and other greenhouse gasses did rise to current atmospheric levels and even higher, but not at the speed we see today.

If we reanimate Darwin and send him on a new Beagle expedition to reassess the biodiversity 200 years after

I love it. This should be a new scientific imperative.

    Watsisname wrote:
   But it is a pretty serious issue, and we face a wide variety of challenges because of it.


Sure, it can be framed that way, but does Earth benefit from all the attention it gets?  Climate change is portrayed, with much help of the scientific community, as the main environmental challenge of today.  That, I think, is a serous issue.  Because, frankly, it isn't the main challenge.  If the wild animals of the world could vote between a world at 1850 temperature with our 8 billion presence and a 3 degree warmer world, gradually increased over 250 years, without us, what would it be?

Erm, maybe I'm just dumb, but what exact argument are you making here? Climate change is a VERY far-reaching issue, with repercussions we don't even know the extent of. Who cares what the animals think (or the 'planet') - they are not the ones with the science to measure their effect on the environment, nor possess the tools to possibly fix it. We are. 99% of all species on this planet have died in the past, mostly from climate-change events. This is nothing new, and life will go on. But we might not if we don't get our rears in gear.

We actually do know when CO2 levels rose and how far in the past.

Yes, but the time resolution is low.  The transitions can be smooth or bumpy.  There might be blips that we don't see. Just because we don't have the data doesn't mean it didn't happened.

Climate change is a VERY far-reaching issue, with repercussions we don't even know the extent of. Who cares what the animals think (or the 'planet') - they are not the ones with the science to measure their effect on the environment, nor possess the tools to possibly fix it. We are. 99% of all species on this planet have died in the past, mostly from climate-change events.

And I wonder what exact argument you are making here.  Do extrapolations, knowledge and the ability to measure something makes it worse or more real?  This sounds like the hypochondria I was referring to.  

Yes, but the time resolution is low.  The transitions can be smooth or bumpy.  There might be blips that we don't see. Just because we don't have the data doesn't mean it didn't happened.

So your counter is essentially an argument from ignorance? If indeed we need to wait for more evidence that climate change hasn't happened in the modern context, then we will be waiting for a long time. Although I agree with you that global warming probably won't kill us off, it will be immensely damaging to our society. I state that without entailing hype and click-bait.

   Stellarator wrote:
   Climate change is a VERY far-reaching issue, with repercussions we don't even know the extent of. Who cares what the animals think (or the 'planet') - they are not the ones with the science to measure their effect on the environment, nor possess the tools to possibly fix it. We are. 99% of all species on this planet have died in the past, mostly from climate-change events.


And I wonder what exact argument you are making here.  Do extrapolations, knowledge and the ability to measure something makes it worse or more real?  This sounds like the hypochondria I was referring to.  

I am not trying to stir the pot, nor have I taken to wailing in the streets that we're all going to die. I honestly don't think we CAN reverse the effects of climate-change at this point - it is just something we're going to have to shoulder and carry on with. Technologies like CO₂ filtering, proper waste disposal and clean energy sources can only help at this point.

However, I will point out that imposing a sense of false moderation on the issue because we are afraid of making grandiose predictions, isn't a healthy way to broach a possible co-existance and mediation of the issue.

Yes, but the time resolution is low.  The transitions can be smooth or bumpy.  There might be blips that we don't see. Just because we don't have the data doesn't mean it didn't happened.

Time resolution isn't always low.  In some areas it is as good as annual or even seasonal, thanks to varves.  These have actually been used to reveal the evolution of the Moon's orbit billions of years ago, by recording the tides and thus ratio of day length to lunar month!

More to the point though, the geologic record is of course not complete, and I have specifically referred to the known record.  However, known Earth history is still pretty vast and well understood.  More importantly, and as I'll show, the Earth cannot experience a greenhouse gas induced warming of a few Celsius over a similar or shorter timescale as what we are going through now, and not leave clear evidence behind.  The reason we do not see such changes in the record is not because they do not get recorded.


Natural greenhouse gases have long residence times in the atmosphere.  Chief among them are CO2 and methane, with methane being converted to CO2 by reactions with hydroxyl.  (We might wonder about water vapor, but its amount is well fixed by the Clausius-Clapeyron relation.)  These greenhouse gases might be injected rapidly, but they cannot go away rapidly!  They stick around and warm the planet for tens of thousands of years.  The duration of the warmth changes precipitation patterns, and therefore also sedimentation processes.  Furthermore, with carbon being injected to the atmosphere, which is then absorbed into the oceans, there is extinction to carbonate-shelled organisms.  This again shows up in the sediments.

Indeed, the PETM itself is a beautiful proof.  Greenhouse gases were injected, global climate warmed and stayed warm for millenia, and we see the effects plainly in the rocks.  Not just in a few places, but all over the world.  It so distinct and widespread that it serves as a golden standard stratigraphic marker, which geologists used for demarcating the boundary of two eras of Earth history long before they even understood what caused it.


Dramatic change in sedimentation processes at the onset of PETM in Sinai.


Pacific sediment cores showing benthic foraminifera extinction during PETM (marked by the black arrows).


Pinkish strata of a PETM flood plain seen in Big Horn Basin, Utah.


Besides greenhouse gas emissions like during the PETM or today, can anything warm the world this much this rapidly?  Probably no. 

• Volcanoes cool the planet rather than warm it, and the effects go away within a few years.  Likewise with asteroid strikes.
• The Sun can change its output rapidly, but when it does it is not by very much.  When it changes a lot, it does so very slowly.  
• Earth's albedo may change, but not very quickly.  The fastest is probably during the ends of snowball earth episodes.
• Earth's orbit and axial tilt change, but on well understood cycles with 10s to 100s of thousands of year periods.
• Faster temperature change may be seen in Greenland ice core data, but these are not global rates.

So your counter is essentially an argument from ignorance?

I don't think you get my point.  I'm trying to point out that claims about modern changes being greater than any time in Earth's history are extremely bold statements, since Earth's history is ridiculously long compared to a modern period of decades, and the geological records have low temporal resolution and are often incomplete. 

More importantly, and as I'll show, the Earth cannot experience a greenhouse gas induced warming of a few Celsius over a similar or shorter timescale as what we are going through now, and not leave clear evidence behind.  The reason we do not see such changes in the record is not because they do not get recorded.
[...]Indeed, the PETM itself is a beautiful proof.  Greenhouse gases were injected, global climate warmed and stayed warm for millenia, and we see the effects plainly in the rocks.

Currently the estimate is that we've experienced a greenhouse induced warming of about 1 C over 150 years, or around 0.6 C over the past 70 years, and we need to be clear whether we speak of the facts as of today as unprecedented or the projections.  I also would appreciate if you could clear up what you say above.  First you say that rapid warming due to greenhouse gases cannot have happened in the past because we would have seen it, then you seem to say that PETM was one such incident.  That seems contradicting.

We should also clear up whether the cause of warming is relevant to the effects of the warming.  Do we see more or less extinction depending on whether the cause is mainly CO₂, or methane, or solar insolation, or human causes?  Am I wrong in trying to focus on climate change itself in this discussion?  

As for evidence for mass extinctions in the geological record associated with climate change, I think it should be considered that such events may be more complex.  Was the climate the only thing that changed?  Or did a combination of changes cause the extinction?  Or was the climate change merely a symptom of something else that was the real killer.  And, certainly, do most warming events of 3+ degrees cause extinctions or is it the exception?

Faster temperature change may be seen in Greenland ice core data, but these are not global rates.

So, Greenland shows shifts of something like 20 degrees C.  And even great variation deep within ice ages.  Globally the estimates seem to indicate swings of 4 to 7 degrees, i.e. much larger than most estimates of the current warming projections.  Ice ages are global events, which is evident by geological evidence of tropical glaciers, sediments, etc.  I don't think we know the exact decadal rates of these changes.  If 0.2 C per decade, the projection for the near future, is very bad or catastrophic, it would be interesting to know if it has ever happened before, but considering how unstable ice ages are and the length of Earth's history, I would be very surprised if it were proven that global warming of 0.2C per decade or 2C over a century will be unprecedented.

I'd like to add that I've observed Svalbard first hand warm about 5C over two decades during more than a dozen visits.  Apparently it should have scared me, but I find it fascinating instead.  It's like geology in fast forward.  The changes since the 90's are similar to those requiring centuries or millennia elsewhere in the world. There was a warming of about 5C from 1915 to 1935 as well.  Scientists then weren't alarmed (other than getting forced to double check whether the meteorological data were correct), but clearly fascinated.  The mood now is quite different.

First you say that rapid warming due to greenhouse gases cannot have happened in the past because we would have seen it, then you seem to say that PETM was one such incident.  That seems contradicting.

You suggested previously that just because we don't see "short blips" of warming in the climate record does not mean they did not occur.
 
What I am showing is that short blips of greenhouse-forced warming cannot happen without being recorded, precisely because the effects of greenhouse gas emissions cannot be short lived.  If a lot of greenhouse gas is injected in the atmosphere rapidly, the result is many thousands of years of global climate change at least, which leaves behind clear evidence in the rocks.  The PETM is a proof that the effects get recorded, and are obvious.  It is also approximately the shortest blip of warming that physics and the dynamics of carbon sources and sinks allows.  Human emissions are more rapid by about a factor of 10, but again the effects will be long lived (unless we do some intensive carbon sequestration).

In other words and very simply, we can be very sure there are not large rapid and brief warming episodes missing from the record.  Physics and geology don't work that way.

We might wonder if other effects besides greenhouse emissions may warm the globe this much this quickly.  We know of many other things that can change global temperature, but none work this much this rapidly in the positive direction.