Arctic's sub-tropical past

rogers_buck · Sep 8, 2004

http://www.ananova.com/news/story/sm_1094301.html?menu=news.scienceanddiscovery

Maddad · Sep 8, 2004

What is the reason you posted the link? Did you have a comment on the article?

earthseed · Sep 11, 2004

What is interesting is that none of our climate models can account for such a warm Arctic Ocean. Also interesting is that equatorial areas were very little wamer than they are now.

rogers_buck · Sep 11, 2004

What a contrast with snowball earth. Makes you wonder.

Maddad · Sep 12, 2004

The snowball earth theory takew us back to 6 or 700 million years ago, more than ten times as old as this. The theory is controversial. Although there is evidence to support it, there is also evidence that is incompatible with it. The earth as a whole has periodically warmed and cooled over the eons, the continental plates have slip-slid around the surface, and the poles have wandered. What is tropical now may have been artic then, and what was artic before could be tropical now.

earthseed · Sep 12, 2004

The warm conditions in the polar areas in the Cretaceous (90 million years ago) and Eocene (55 Mya) cannot be explained by latitude changes due to plate movements, because the amount of movement was not all that much, and the direction of movement is mainly east-west in the arctic. See these maps for the Cretaceous and Eocene periods. However, plate movements did open and close sea lanes, which could profoundly affect ocean circulation, and thus climate. This article from Science Magazine discusses the warm Cretaceous climate.<blockquote>However, computer simulations of Cretaceous climate indicate that radiative warming caused by increased greenhouse gas concentrations (principally CO2) were more important than paleogeography in explaining Cretaceous global warmth. Estimates of Cretaceous pCO2 generally range from four to eight times preindustrial values, and some intervals, such as the Turonian-Coniacian, may have exceeded this amount severalfold (perhaps explaining the warming spike observed for that time). Climate models have revealed, however, that although CO2-induced warming can approximate globally averaged temperatures for the Cretaceous, the models predict steeper latitudinal temperature gradients (both warmer tropics and colder poles) than geologic data seem to allow. This has led some to suggest that the oceans played a greater role in transporting heat from the tropics to the poles than they do today, particularly through sinking of dense, saline waters formed in restricted low-latitude basins. However, Sloan et al. calculated that doubling the ocean heat transport to balance the energy budget for the warm climate of the early Eocene would require a mechanistically prohibitive poleward flow of warm, saline water masses. These authors concluded that either the oceanic proc</blockquote>

paleo · Sep 12, 2004

This is no reflection on the poster (the more postings the better) As a geologist and paleontologist I would put the value of the article on the same level of 'earth-shattering' breakthroughs as : 'Archaeologists find evidence of man in America before Europeans." and: 'The Sun is really a star !!' Good points above about 'the past'. Last year, 10 million years ago, 50?, a billion? General statements about the past are like asking a question: 'what's the temperature?'...of course, you'd have to ask 'of what?' The Sun? The air outside my window? The melting point of lead? The Arctic ten thousand years ago was not the same as the Arctic 50 thousand ago, or 50 million ago...or 4 billion ago. No real relation to proving anything. The seas, continents, core temperature and everything else was different so the Arctic today compared to the Arctic at any time more than a few thousand years ago is comparing apples and oranges.

earthseed · Sep 12, 2004

paleo, the article is old news, but it still is significant to know the Arctic Ocean was warm not so long ago in geological terms. Reconstructing the climate of the past is the only way we will be able to predict the future. That is what paleontology is for, is it not? stevehw33, one lesson from the past, as you point out, is the Earth's climate has changed significantly without human causes. I do not think the case for the sun being a major cause is well established; there are many other inputs as well, such as Milankovitch cycles. Another lesson is that climate is unstable, and change can happen quickly. The historic changes you mentioned are small compared to events during the past tens of thousands of years. Human changes in the amount of greenhouse gasses, as well as changing the Earth's albedo (reflection of sunlight) and particulate matter in the atmosphere are not that great, but they could trigger a major climate change event.

silylene old · Sep 12, 2004

steve33: To add to your post of recent significant climatological changes: 1) the Nile river in Egypt actually froze over in 829 and in 1010A (the 1010 A.D fact is rather odd, since 1000 AD was a warm period in the North Atlantic. Was there a significant volcanic eruption that year?). 2) from 1857-1875, Cape Cod Bay completely froze every winter, sometimes not melting until late March. On a few of those years, it was actually possible for one to ride a horse from Boston to Provincetown across the Bay. (nowdays, significant sea ice in Cape Cod Bay is unusual) 3) During the "little ice age" of the 1300s-1600s, snow and ice were permanent year-around and accumulating on both the Scottish and the Labrador highlands. Glaciers grew significantly in the Northwest N. America, the Alps, Patagonia and African peaks which no longer even see snow. We were dangerously close to tilting the Earth's albedo high enough to enter a new long-term ice age. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

Maddad · Sep 12, 2004

paleo It's my policy to disagree with anything you say, but in this case I'll have to make a painful exception. You're exactly right. After a five second glance at earthseed's cretaceous map, the reason for the warmer climate is obvious. It shows the sea level about 100 meters higher than it is today. With the global ocean curents better able to distribute low latitude warm water to polar regions, the ice caps have vanished. This darkens the planet's albedo, so it doesn't radiate as much long solar rays back into space. No wonder the world was warmer then. What I can't figure out is why this author considers these simple mechanics to be mysterious and unexplainable. I guess he just fell asleep in geology class that day.

aaron38 · Sep 13, 2004

Silylene, "We were dangerously close to tilting the Earth's albedo high enough to enter a new long-term ice age." Being very close to a tiping point often amplifies the effects of otherwise small things, the butterfly effect if you will. Could have the industrialization beginning in the 1600s actually tipped the balance and prevented the ice age? Pushed us back from the brink? Could we actually owe this favorable climate to our own activity?

earthseed · Sep 13, 2004

Hi, maddad. Taking another look at the Cretaceous map, it does appear to be an almost perfect hothouse world. The key point is water absorbs solar heat better than land. Land masses at the poles cause cooling because ice build-up lowers the level of the oceans. Land at the equator causes cooling because that is where sunlight is the most intense, thus more warming potential is lost. The Cretaceous world has most of its landmass in the temperate zones, with multiple wide channels to the polar regions to distribute ocean warmth.

silylene old · Sep 13, 2004

Could have the industrialization beginning in the 1600s actually tipped the balance and prevented the ice age? Pushed us back from the brink? No, not from 1600. But from 1850, maybe. But I do think that industrialization beginning from 1850 onward might have helped pull us from the brink. Look at graphs of global temperatures, global CO2 (and methane) vs year. You will see that despite an rapidly increasing concentration of greenhouse gases from 1850-1920 and 1940-1970, the global temperature remained constant (except for a distinct rapid warming during 1920-1940). I do wonder, had the global greenhouse gas concentration not been increasing, that the period from 1850-1970 may have been a period of decreasing global temperatures, with a stable time from 1920-1940. And the (potential) 1850-1970 could've-been-cold period would have been arrived just after a stable, but cold period from 1780-1850. I would like to see a modeled climatological retrospective from 1850-1970 without greenhouse gas, beginning at the 1850-earth-albedo. I do wonder what the effect would've been. Here is some data to ponder: http://www.tamug.edu/labb/Global_Warming_Info.htm <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

Maddad · Sep 14, 2004

earthseed "Hi, maddad. Taking another look at the Cretaceous map. . . The key point is water absorbs solar heat better than land. Take another look? What for? That's what I said. "Land masses at the poles cause cooling because ice build-up lowers the level of the oceans. No. "Land at the equator causes cooling because that is where sunlight is the most intense No. silylene "had the global greenhouse gas concentration not been increasing, that the period from 1850-1970 may have been a period of decreasing global temperatures The second half of the 1800's is when we came out of a mini-ice age. Your speculation suggests that maybe we did because of industrialization greenhouse gasses. You might be right.

JonClarke · Sep 15, 2004

One has to be careful about generalisations even in the Cretaceous. Although the Cretaceous poles were always warmer than present, this does not mean they were always tropical. For example, at palaeolatitudes of 60-80 degrees S in Australia in the Early Cretaceous there is abundant evidence of frigid conditions - ice rafting, cryoturbation, episodic ground freezing, and formation of glendonites. At the say time there were forsts and a range of terrestrial animals, some quite large (Muttaburrasaurus, Allosaurids), and a wide range of marine animals, some very large (Kronosaurus) Cheers Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

Maddad · Sep 15, 2004

Jon! Great to see you again. Since we last spoke I took a summer four credit Earth Science course as an elective in my nursing program. I decided to take it as a result of talking with you.

earthseed · Sep 16, 2004

maddad: About the Cretaceous climate and the continental configuration, I was agreeing with you. Sorry if it seemed otherwise. But about the next two points:<blockquote>"Land masses at the poles cause cooling because ice build-up lowers the level of the oceans." No. "Land at the equator causes cooling because that is where sunlight is the most intense" No. </blockquote> Can you explain why these statements are not correct?

earthseed · Sep 16, 2004

Jon: I guess that polar climates would be unstable, from the evidence you give. If the polar warmth was due to ocean currents, an El Nino type of cycle could play havoc with the climate. Along with maddad, I would like to say that your patient replies to my sometimes naiive questions stimultated my interest in geology, and I have since done a lot of reading about it. I certainly do not look at rocks the way I used to!

silylene old · Sep 16, 2004

"Land at the equator causes cooling because that is where sunlight is the most intense" Currently the cumulative daytime for land in the far northern hemisphere is about 3% more than the cumulative nighttime, compared to land at the equator or land in the southern hemisphere. To some extent this is a matter of definition. But the north also gets more cumulative solar irradiation than the southern hemisphere due to the ellipitical shape of the earth's orbit which (currently) makes winter occur at perihelion. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

JonClarke · Sep 17, 2004

Thanks, you have made my day. I can transfer to others at least a fraction of the joy that understanding the world we live it brings me, then I am very content. Certainly the Cretaceous world would had all sorts of interesting climate events like the Southern Oscillation (El Nino/La Nina) and monsoons. However our climate models are still too rough to be able to get that sort of detail, although they are starting to get there. Some other things to think about during the Cretaceous. 1) The ocean was often strongly stratified like the Black sea is today, leading to widespread black shales (source rocks for petroleum). this means that the modern thermo-haline circulation was absent. 2) Because there were no ice caps and the ocean ridges were elevated sea level was quite high for much of the Cretaceous and land areas were quite small compared to the present (20-30%). However these epicontinental seas have few counterparts today, those we do have (Baltic Sea, Hudson Bay, Gulf of Carpentaria, Persian Gulf, Sunda sea) are much smaller. 3) The tropical areas were probably too warm for coral reefs. Tropical reefs there were but they were composed of calacerous algae and the bizzare rudist molluscs. Cheers Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

Maddad · Sep 17, 2004

earthseed "Land masses at the poles cause cooling because ice build-up lowers the level of the oceans. Land at the equator causes cooling because that is where sunlight is the most intense Can you explain why these statements are not correct? The ice build up is the cause of ocean level dropping more than the ocean level dropping causing ice build up. The second statemet is "ubhaya assidham", Sanskrit for not logical from any point of view. There isn't more sunlight at the equator, but rather the sunlight you have is more direct and thereby more concentrated. It is not cooler at the equator than the poles; it is warmer. Land anywhere, either equator or at the poles, experiences greater fluctuations in temperature than water does. On average land would have a higher albedo than water, so it would be more reflective than water and thereby cooler, but this has nothing to do with latitude. polarwander The wandering of the North Pole isn't something that happens suddenly. We think it took a half billion years, 500 million, to get where it is from about where the Hawaiian Islands are now. The exact mechanism is still debatable, but the crust of the Earth is fractured into a dozen or so major pieces which slip-slide around at up to maybe 15 centimeters a year, usually more like 2 to 5 centimeters per year. Those speeds don't give you much sudden movement.

JonClarke · Sep 17, 2004

Hi Polar Determining palaeolatitude is a challenge. There are four methods that can be used in the Cretaceous. One is looking at sedimentary environments for evidence of cool or cold climate deposition features - dropstones, cryoturbation, glendonites etc. You can also find the tropics by the distribution of warm water features like abundant reef buildups, and the subtropics by evidence of deserts and evaporites. The second is to look at the oxygen isotope record and plot the mean annual temperature. The third is to look at the biota for features indicative of cool climates and/or extreme seasonality. Plant leaf morphology is one of the best, at least for the Cretaceous. The problem with all these methods is that the locations the data come from have moved because of plate tectonics. This comes to the fourth method. While we can't directly measure the position of the rotational pole we can plot the positions of the magnetic poles. This, with constraints from the patterns of seafloor spreading, allows us to reconstruct the continents to a particular era. when this is done all the high latitude features - sedimentary, geochemical, and palaeontological, all end up at very high magnetic latitudes. Conversely, evidence to vaporites, reefs and deserts end up at low to moderate latitudes. So we can say for the Cretaceous at last (and probably back to the beginning of the Phanerozoic) that there has been close coincident (with 20 degrees at any rate) between the dipole and rotational axes. That this means is that the north rotational pole was north of Alaska and the south pole was on the edge of Antarctica near where it joined Australia. What is now Yucatan was close to the Cretaceous equator. The impact happened on a thick sequence of shallow marine warm-water carbonates and evaporites. Cheers Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

newtonian · Sep 18, 2004

You all - I have my own favored model which involves milder climates in the arctic and antarctic. 1. The vast carbonate deposits in earth's crust. It is generally accepted that these were deposited by the geological carbon cycle, which involves liquid oceans and dissolved CO2 (= carbon dioxide) from the atmosphere, plus 'metalic' ions also in the oceans (Calcium, Sodium,...). Extrapolate back in time and we end up with an atmosphere like Venus today- without the sulphuric acid. 2. Geology indicates a hot origin - too hot for liquid water at present atmospheric pressure - but perhaps not at the ancient atmospheric pressure. 3. Catastrophism vs. uniformitarianism. I doubt the changes from that past to the present conditions were all gradual and uniform, either on earth or on Venus (which went in reverse direction compared with earth). 4. Accretion at origin - Biblical model - Job 38:9. Early earth had thick clouds (perhaps including much dust, perhaps including carbon) producing gloom on earth's surface, perhaps even less light than Venus today. 5. Condensation catastrophes, causing fossilization, among other things. As earth's crust cooled some water would condense, dissolving some CO2, lowering atmospheric pressure and lowering the boiling point and causing evaporation. That is a cycle in a process. Now, the above is partly theoretical, and must be modified by the geological evidence, of course. 6. The last condensation catastrophe would be the Biblical flood of Noah'sday to which virtually all cultures testify in their ancient history. 7. Before that catastrophe, earth's atmospheric (perhaps in the thermosphere, perhaps way beyond at La Grange points, etc.) water vapor was much thicker, and this produced a greater greenhouse effect which caused the polar climates to be tropical. 8. Continental Drift- catastrophic version. The plates were sent in violent mot

earthseed · Sep 19, 2004

Jon:<blockquote>Some other things to think about during the Cretaceous. 1) The ocean was often strongly stratified like the Black sea is today, leading to widespread black shales (source rocks for petroleum). this means that the modern thermo-haline circulation was absent.</blockquote>I understand that the temperature difference between the equator and poles was smaller during the Cretaceous than now. Yet the evidence seems to be ruling out large scale ocean current transfers of heat. Going back to the article in Science Magazine I quoted from earlier, they claim high carbon dioxde levels are the heat source:<blockquote>However, computer simulations of Cretaceous climate indicate that radiative warming caused by increased greenhouse gas concentrations (principally CO2) were more important than paleogeography in explaining Cretaceous global warmth. Estimates of Cretaceous pCO2 generally range from four to eight times preindustrial values, and some intervals, such as the Turonian-Coniacian, may have exceeded this amount severalfold (perhaps explaining the warming spike observed for that time).</blockquote> But is the CO2 a cause or an effect? Today there is about 50 times the amount of dissolved CO2 in the ocean than in the atmosphere. Given that the solubility of CO2 decreases with water temperature, one would expect CO2 concentration to track global temperature even if it had no greenhouse effect at all. Carbon dioxide is a greenhouse gas, but the effect is less than linear. From this site of The Geophysical Fluid Dynamics Laboratory (part of NOAA), we find that <blockquote>...net planetary radiative forcing changes roughly linearly in response to logarithmic changes in CO2. Thus, a quadrupling</blockquote>

JonClarke · Sep 19, 2004

Hi earthseed Three words: we don't know. There are lots of hypotheses out there to explain the more equitable temperature distribution and probably warmer global average temperatures during the Cretaceous. There may be some truth in all of them at differing times in earth history. The problem is testing the hypotheses. Given the complexity of climate modelling, the limits of our knowledge of the Cretaceous and the problems climate modellers have in modelling today's climate, these questions are difficult ones to answer. Even demonstrating that the south pole was cool enough to have seasonal ice in the early Cretaceous took decades to achieve, despite some of the evidence, i.e. dropstones being known for 50 years or more. Cheers Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

Arctic's sub-tropical past

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