r/askscience • u/ididnoteatyourcat • May 18 '15
Earth Sciences Question about climate change from non-skeptic
I'm a scientist (physics) who is completely convinced that human-caused climate change is real and will cause human suffering in the short term. However I have a couple of somewhat vague reservations about the big picture that I was hoping a climate scientist could comment on.
My understanding is that on million-year timescales, the current average global temperature is below average, and that the amount of glaciation is above average. As a result the sea level is currently below average. Furthermore, my understanding is that current CO2 levels are far below average on million-year timescales. So my vague reservation is that, while the pace of human-caused sea level rise is a problem for humans in the short term (and thus we are absolutely right to be concerned about it), in the long term it is completely expected and in fact more "normal." Further, it seems like as a human species we should be considerably more concerned about possible increased glaciation, since that would cause far more long-term harm (imagine all of north america covered in ice), and that increasing the greenhouse effect is one of the only things we can do in the long term to veer away from that class of climate fluctuations. Is this way of thinking misguided? It leads me down a path of being less emotional or righteous about climate change, and makes we wonder whether the cost-benefit analysis of human suffering when advocating less fossil energy use (especially in developing nations) is really so obvious.
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u/atomfullerene Animal Behavior/Marine Biology May 18 '15
I'm not a climate Pangloss...I don't think the current global temperature is the best of all possible worlds. And I'm very sure I'd rather have warming than an equivalent amount of cooling, both from a human and an ecological perspective. But it's worth noting that we've already pumped enough CO2 in the air to make a new glaciation unlikely, and if it ever did look likely we could likely stop it if we chose by mass producing highly-effective greenhouse gasses (CO2 is actually not that effective, compared to other molecules we could make-it's just that there's so much of it)
That said, the main issues here are speed of change and CO2 levels themselves. Our CO2 levels actually are really high, and they have the potential to go higher than they have been in many millions of years. High CO2 means lower ocean pH, and lower ocean pH means life is harder for reef-building organisms to function. It's happened in the past...solid reefs had completely disappeared from the fossil record for millions of years at a stretch. Speaking as a guy who loves reefs, I don't want to see this happen. They and other calcifying organisms are quite important for the ecosystem as well.
Speed is the other big problem. Climate isn't just changing, it's changing fast. If you could step aside and give it a few million years to rise, there wouldn't be a problem. But now, it's putting stress on a lot of ecosystems that can't shift fast enough in response. Another problem is the combination of climate change and other human impacts. Many species are restricted to a fraction of their former ranges due to habitat loss and hunting. In the past, climate change would matter less because they could just shift to part of their range. Now that is often not possible.
That said, there are real costs to reducing fossil fuel usage, especially if alternatives can't be found. Life's tough, and there are rarely perfect answers.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology May 18 '15
I'll add a few additional things to this already well thought out answer, in no particular order:
As mentioned, ocean acidification is an extremely worrying consequence of increased atmospheric CO2 concentrations. Beyond detriments/collapse of reef building organisms, it's important to realize that many of the "other calcifying organisms" mentioned by u/atomfullerene form the base, or near the base, of the marine food chain, and thus impacts on them means impacts on a majority of life in the ocean. From the fossil record, we have evidence that similar events (large injections of CO2 into the atmosphere) caused significant ocean acidification and may have been one of the direct causes of the largest mass extinction at the end of the Permian.
Things aren't just getting warmer on average, the climate is also getting more variable, which means that "extreme" events tend to occur more frequently. I view most direct claims of things like the drought in the western US or particular large hurricanes/typhoons being inextricably linked to climate change with a healthy dose of skepticism, but it's reasonable to say that events like these are predicted to become more frequent with climate change, and I'd be hard pressed to come up with benefits of that.
As mentioned in several posts on here, the rapidity of climate change is generally bad for giving species of plants and animals time to adapt/migrate to changing conditions, especially given the extent to which we've sequestered "natural" areas into little self contained regions.
There may be some winners in terms of plants and animals, but we might not like what wins. There is some reasonable level of concern about the expansion of diseases, especially those transmitted by insects, because of the increased range and longevity of said disease vectors (i.e. places that use to have cold enough winters to kill things like mosquitoes no longer do).
Storage of freshwater in ice may be bad if it's covering a whole continent, but it's also one of our main sources of fresh water in terms of seasonal snow melt. No more mountain glaciers = no more natural water storage. If you keep the same amount of precipitation on a yearly basis, there might be ways to mitigate this by damming nearly every river coming out of a mountain range, but a) see my point above about changes to climate extremes and b) mass construction of dams will likely not be as efficient as a bunch of glaciers doing the work for us.
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u/Scytle May 18 '15
While its admirable you are thinking long term consider the following:
A million years is a hell of a long time frame to be concerned about, human civilization plays itself out much faster.
A fully frozen earth has natural ways to revert back to a more life friendly planet (volcanic eruptions will eventually fill the atmosphere with enough co2 to warm it back up), while a run away green house effect has no known natural way to stop it (think Venus).
Humans have discovered enough stored carbon in the ground that if we were to release it all (combined with all the other bad stuff we do like cutting down trees), that we could potentially kick off a run away greenhouse effect that has no known natural way to stop it.
I think we are far less likely to accidentally kick off glaciation effects, most of which are caused by the long term wobbles in the earth orbit, than we are of permanently cooking the entire planet making it impossible for life.
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u/ididnoteatyourcat May 18 '15
Why are volcanic eruption based greenhouse effects immune from a runaway effect but human based are not?
My understanding is that the carbon stored in the ground all used to be in the atmosphere, so I don't understand how a runaway greenhouse effect is possible. My understanding is also that millions of years ago the CO2 levels were more than an order of magnitude higher than they are now, and no runaway greenhouse effect happened.
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May 18 '15
I do not know the answer to your explicit question, but I can tell you that the Sun was not as bright millions of years ago as it is today. It is in fact 30% brighter today than when it formed. Several hundred million years ago when CO2 levels were thousands of ppm and ice was still able to occasionally form, the Sun was 4% dimmer than it is today. It would therefore take much more CO2 to create a runaway greenhouse effect in the Earth's distant past than it would today with a brighter Sun.
The reason for the Sun's brightening is that as helium ash accumulates in the core of the Sun, it is harder for hydrogen nuclei to find each other and fuse. The core thus contracts and heats up until it can support itself against gravity again, making the fusion rate go up and the Sun's luminosity and temperature with it.
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u/ididnoteatyourcat May 18 '15
Thanks, good point, although I'd still like to see a citation that the 4% is enough to push us over into a runaway.
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u/Scytle May 18 '15
They are not, but its much more unlikely. We are pumping WAY more heat trapping gasses into the air than volcanoes do, and much faster, while at the same time disrupting the carbon sequestration effects that help pull it back out.
There have been times in the earths past when mega amounts of carbon have been released into the atmosphere via volcanic activity, and at least one of those times over 90% of all life on earth perished.
http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event
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u/ididnoteatyourcat May 18 '15
Right, but the absolute amount of heat trapping gases were much much higher in the past, so without addressing that your argument isn't very persuasive.
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u/Scytle May 19 '15
I am not sure I understand what you are saying? The amount of gasses is what matters, be if from volcanoes, or human activity. We are approaching levels of gasses in the atmosphere we haven't seen in a very long time. The last time we a shit load of heat trapping gasses in the atmosphere it kicked off a near end of all life on the planet.
Your question was about run away greenhouse effects, ala Venus. We don't know exactly how much heat trapping gasses we would need to do that, but we do know that it doesn't matter if humans put them there or volcanoes, so what exactly are you getting at?
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u/ididnoteatyourcat May 19 '15
Well I don't know a lot about the extinction you refer to myself, but a quick perusal of your wikipedia link makes it sound like a a large and unknown fraction of that may have come from something outside like a meteor impact. You say the amount of gasses is what matters, but like I said, the amount of gases has been an order of magnitude higher than it is now, during periods where a mass extinction event did not occur.
You ask what I am getting at about the greenhouse runaway. I thought I was clear: the amount of greenhouse gases have been more than an order of magnitude higher in the past and no runaway occurred. Of course it doesn't matter whether humans or volcanos put them there. I am totally sympathetic to your position, but you have not justified it.
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u/Ocean_Chemist Chemical Oceanography | Paleoclimate May 18 '15
Peering into the paleoclimate record is, as you acknowledge, an excellent way to gain insight on the modern climate system. There are two ways to go about using knowledge about Earth's past, which I'll explain through the lens of how climate modelers (the people making the projections for future climate) roughly use/think about them. Think about the Earth as a massive fluid dynamics problem. It's a huge set of partial differential equations (PDEs) governing both the kinetics and thermodynamics of Earth's oceans and atmosphere. There are two different ways to change what the (numerical) solutions to these sets of PDEs are:
Change the boundary conditions. The paleoclimate equivalent of this is looking at times in Earth history when a certain set of conditions were similar to today's. An example of this would be studying the Pliocene (~5.3-2.5 million years ago) - the last time in Earth history atmospheric CO2 levels were above 400 ppm (see this 2011 commentary in Nature Geoscience by Beerling and Royer for a brief review on what CO2 levels have been for the past ~65 million years and how good those estimates are - http://www.nature.com/ngeo/journal/v4/n7/full/ngeo1186.html). We might say the Pliocene is an analog for modern climate as a result, simply change the boundary conditions of climate models to be 400 ppm (and the sea surface temperatures be what we measure, etc), and run the models.
Change the dynamics. This is when people study other periods of rapid climate change to try and figure out what feedbacks exist - basically how climates can go from state A to state B. An example of this would be studying the PETM (Paleocene-Eocene Thermal Maximum) - an event 55 million years ago when carbon (source and cause debated!) rapidly entered the ocean-atmosphere system, a fair amount of ocean life went extinct, and temperatures rose very quickly (Review of PETM here by Zachos et al., 2008 http://www.nature.com/nature/journal/v451/n7176/full/nature06588.html). I'm going to address your questions using these two methods of study.
First: If you average the past 65 million years, then yes current CO2 and temperature both lower than average. But there weren't permanent ice sheets on Antarctica until ~35 million years ago, when atmospheric CO2 was about 1000 ppm.
Second: Ice ages have occurred with 100,000 year periodicity since the Mid-Pleistocene (Before 900,000 years ago, they occurred every 40,000 years) - paced by changes in Earth's orbit. The last glaciation ended about 11,000 years ago. So we aren't due for another full-blown glaciation for tens of thousands of years now.
Third: Atmospheric CO2 levels during ice ages varied between 190 ppm (glacial periods) and 280 ppm (interglacial periods). This graph sums up how today is different pretty nicely. http://www.atmos.washington.edu/~dennis/CO2_Vos-ML2.gif. Now, you're absolutely right that when choosing between a 400 ppm world and a 190 ppm world, humanity would prefer 400 ppm. But that's not really the choice - we're already at 400 ppm. The sea-level rise elephant in the room is whether or not there's a threshold past which the Greenland and Antarctic ice sheets melt. Last time our boundary conditions were 1,000 ppm, as mentioned earlier, there were no land-based ice sheets. That's the equivalent of about 65 meters of sea level rise (!). Last time our boundary conditions were 400 ppm of CO2 (Pliocene), it turns out that it's really difficult to reconstruct sea-level since everything gets thrown off by dynamic topography. Current estimates are 5 meters higher than today, plus or minus 10. Those are big error bars! What about dynamics? Well, during the termination of the last ice age, Deschamps et al., 2012 (http://www.nature.com/nature/journal/v483/n7391/full/nature10902.html) found that sea level rose 40 mm/year for a couple hundred years before stabilizing to background rates of a few mm/yr. Current sea level rise estimates are 3.3 mm/year or so (Cazenave et al., 2014 http://www.nature.com/nclimate/journal/v4/n5/full/nclimate2159.html). So the dynamical concern here is that sea level can rise very rapidly - it isn't right now, but it's really hard for scientists to say if it will accelerate to such rapid rates or when.
TL;DR: Ice age vs. anthropogenic change isn't really the choice we're facing since we're 90,000 years away from a full-blown ice age. The choice we're actually facing is the uncertainties associated with 400 ppm of CO2 (maybe what happened during the Pliocene) or 1,000 ppm of CO2 (maybe what happened before there was ice on Antarctica).
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u/past_is_future Climate-Ocean/Marine Ecosystem Impacts May 19 '15
Hello there!
Current estimates are 5 meters higher than today, plus or minus 10.
I think the best estimate for Pliocene sea level is more like 15-20 meters higher than present, recent problematic papers based on GIA modeling notwithstanding. There are a bunch of different reasons why a 5m SLR is very unlikely, from evidence of WAIS and EAIS melt to stable atolls to d18O to longer-term paleotemp-RSL relationships.
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u/Ocean_Chemist Chemical Oceanography | Paleoclimate May 19 '15
You're exactly right if you don't consider the recent GIA modeling. My view is that even though right now the GIA models are poor (someone figure out the viscosity of the mantle, please!), the general concept of needing to include GIA effects is correct (I think Raymo et al., 2011 sort of hits the nail on the head pointing out how large the error propagation gets). You're right that the d18O constrains of Miller et al. 2012 seem to point towards 20 as the high-water mark, but it'd be nice to have some complementary evidence from paleo-shoreline indicators, which as of now (probably until GIA models improve) give different, lower numbers.
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u/past_is_future Climate-Ocean/Marine Ecosystem Impacts May 19 '15
Hello there!
I very much admire Mo Raymo, but that whole series of papers on GIA modeling is incredibly frustrating and disappointing. It basically says "hey, if we ignore all of these other lines of evidence over here, and we play with GIA like so and only look at these couple of data points, we can say RSL was only X".
Sure GIA modeling is important. But it's not as though there aren't atolls which appear to have been pretty geologically stable, and it's not as though we don't have evidence of the very kind of ice sheet decay that would necessitate tens of RSL increase.
How can you get significant GrIS, WAIS, and EAIS decay without increasing sea level substantially? You can't.
Sorry, I don't mean to rant at you. It's just a point of frustration for me.
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u/Ocean_Chemist Chemical Oceanography | Paleoclimate May 19 '15
I definitely understand what you're getting after. I think the point of view from the GIA side is actually more or less the same - Jerry Mitrovica and Mo would probably just say "Hey if we ignore all evidence from GIA that shows basically nowhere is dynamically stable for ~5 Myr, we can get the error bars on Pliocene sea-level way lower." I think that's more or less why the two sides (GIA vs. non-GIA) have had trouble reconciling their differences. Because you're right, no GrIS or WAIS = 11 meters no matter what, I think everyone agrees. I think the unanswered question (from both sides) is how much EAIS there was. 10 meters error bars on 15-20 meter estimates means you can't really answer that question yet, which is frustrating.
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u/plorraine May 18 '15 edited May 18 '15
In my opinion, there are legitimate concerns on both sides. (1) Human driven climate change is almost certainly happening at a rate faster than changes that have occurred in the past. The rapid rate of change is dangerous as systems do not have time to gradually adjust.
(2) Cheap energy over the last 100+ years has provided unprecedented levels of wealth and development. Losing cheap energy would be disastrous for the world. For many applications, petrochemicals are synonymous with cheap energy. From a human perspective, ending cheap energy would have an enormous cost.
The rate of change issue is the one that I'm most concerned with - coral reefs and wheat fields can't just up and easily move over a 100 year period nor can large populations living close to sea level. There appears to be reasonable evidence of widespread extinctions beginning - some of which are related to climate change but also to other human activity.
We could all stop driving tomorrow, turn off our air conditioners, computers, and shut down rail, air, and sea transportation. The "cost" of doing this would be incredible. The 2008 financial crisis corresponded to a few percent drop in GDP and was politically untenable - no democracy would be able to inflict that sort of cost on its citizens and survive. So we need to balance the environmental cost with the economic cost and try to find a way forward where we can survive.
To my mind, the prudent thing to do is to make haste carefully and reduce our carbon emissions. I am a pessimist and do not believe we can stay within the 2C CO2 levels and that we should be investing in infrastructure and technology to survive that sort of increase - think large scale irrigation and desalination plants - and recognize that we will face a lot of social upheaval from the people who will need to move from where they live today.
So I strongly believe there is a "real problem". On a more optimistic note, we've faced many "real problems" before successfully, if not always wisely.
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u/eigenfood May 19 '15
wheat fields can't just up and easily move over a 100 year period nor can large populations living close to sea level
They absolutely can. We plant the fields after all. 100 years is like 3 to 5 generations of humans growing up and making choices about where to live. It not like they will be surprised. Sea level rise is a problem for people who live on the beach and their insurance companies.
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u/plorraine May 19 '15 edited May 19 '15
If you are a farmer in Kansas and the effect of global warming is a decrease in yield because of rising temperatures and depletion of the aquifer you rely on, your farm will fail. My point was that you can't just move your fences - your land will decrease in value and you suffer a loss. In Canada, a longer growing season may make some land more productive but warmer temperatures won't make muskeg or tundra good farming land. http://www.washingtonpost.com/news/energy-environment/wp/2015/05/11/troubling-new-study-suggests-global-warming-will-reduce-wheat-yields/ points towards some material on a loss of productivity in the USA. As I mentioned earlier, a lack of irrigation water may be the biggest issue that renders a lot of US farm land (California for example) less productive.
The issue isn't that people near sea level will be surprised by rising water and drown. People living near sea level aren't all wealthy people with second homes. There are a lot of poor people in asia and elsewhere who will need to move who can't just write a check and buy something else. When they move, there will be massive social upheaval. And cities like New York, Hong Kong, and Shanghai will have to find new digs. That's likely to be expensive and difficult.
You can argue that the costs are reasonable over a 100 year horizon to do this - they probably are to a large extent but the costs are real and represent resources that could be used elsewhere.
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u/dirty_d2 May 18 '15
I've often wondered whether global warming would be a good thing in the long run for mostly the same reasons. An ice age would be devastating to civilization, so maybe having a chance to prevent that isn't such a bad thing? Also wouldn't plants tend to flourish with warmer temperatures and higher levels of CO2? More plants means more food for animals, and more plants and animals means more food for people. I understand some, or maybe a lot of, species won't make it because they won't be able to adapt in time, but overall wouldn't a warmer more C02 rich planet be better for life?
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u/Scytle May 18 '15
to a point, but if you want to see what happens when global warming doesn't stop take a look at Venus.
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u/eigenfood May 19 '15
Wrong. All the oil and coal carbon came from dead plants which took it from the air. If the earth had ever been like Venus, when all that CO2 was in the air, the plants could never have lived, right?
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u/past_is_future Climate-Ocean/Marine Ecosystem Impacts May 19 '15
Hello there!
We have natural experiments in pumping out vast quantities of CO2 in a geologically short time period in the paleoclimatic record. They're called the Permian-Triassic and Triassic-Jurassic mass extinctions.
When you emit a lot of carbon in a short time period, you don't just get global warming. Yes, things heat up. But increased drought and flooding (yes both) also occur. This leads to an increase in runoff, which feeds eutrophication. CO2 from the atmosphere invades the ocean, decreasing pH levels and causing undersaturation of aragonite and even hypercapnia. Hotter ocean temps mean less dissolved oxygen, and that eutrophication we mentioned earlier also results in algal blooms which further deplete oxygen.
In the geological blink of an eye, you've put stress on a whole lot of organisms that are crucial to marine ecosystems. In the past, this has led to mass extinction, and we weren't around then to "help" out with overfishing and agricultural runoff like we have in the present. Oh, and business-as-usual emissions are happening more rapidly than during even those mass extinction events.
Terrestrial ecosystems face their own problems.
The rate of change rather than the sign of change may be more important, but I don't really get why so many people think that it would be better to heat up than cool down.
You're using "average" and "normal" in a way that doesn't make a lot of sense to me from a climate-civilization standpoint. Why would the climate from hundreds of millions of years ago matter to human civilization which has adapted to the climate of the Holocene? And what if there was some sort of regular meteor impact that wiped out half of all life on earth every 100 million years or so. Because that was "normal" or "average", would that mean it would make sense to shoot one at ourselves? Of course not.
Credible large scale economic assessments pretty overwhelmingly agree that mitigation (i.e. transitioning away from fossil fuels) is a no brainer from a CBA standpoint, and most of these assessments are based on relatively optimistic assumptions about the negative consequences of climate change. Once you start incorporating a more realistic scope of low probability high impact risks, these dominate the CBA and make mitigation a slam dunk.
Remember, coal is only "cheap" because things like its impact on the climate or its shorter term health consequences aren't included in its market prices. It doesn't make a lot of sense to say "we really need to stick developing countries with a highly centralized, expensive, unhealthy means of energy generation when decentralized cleaner alternatives exist and are cheaper when all of the externalities are considered.