科学松鼠会

【《财经网》专稿/记者 徐超】在中国数千年的文明史中，王朝的兴衰之谜始终是历史学家乃至普通公众最希望洞察的。有很多理论试图来解释这种以数百年甚至短至数十年的轮回，比如“黄宗羲定律”就指出了农民税负方面的一个怪圈。

中国兰州大学资源环境学院张平中教授及其同事，今年11月7日发表在美国《科学》杂志上的论文却指出，最起码在唐朝、元朝乃至明朝走向衰亡的过程，变化的亚洲季风，起到了推波助澜的作用，甚至给了已处于困境中的王朝以致命一击。

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（本文发表于近两年前的冰点。又一个两年过去，又恰逢冬天，当地天鹅的生活是否有了好转？我不确信，提供另一个角度，供大家思考）

袁学顺在天鹅湖岸边又挖下了一个半米长的坑，他轻轻地把大天鹅的尸体摆弄好，让它的头朝着北方——它飞来的方向。这个冬天，他已经这样亲手埋掉了5只大天鹅。事实上这还远远不是全部。

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原文在这里。作者Ed Douglas，译者Alulu。

译者自述：本人，某生命科学学院中讨生活，坚信人品守恒定律之于人类全体成立，而我的存在就是为世界人民服务。人品不那么低的时候，喜欢自己偷着乐，借点图书馆里最破的书，听点角落里的歌。这次抢稿也算得上一次“偷着乐”，尽管其间纠结数次才搞出这么个勉强能看的东西。。。。还烦请诸位吹毛求疵之。

在电影WALL•E里，人类把地球整的有点乱糟糟。整个星球被塞满了垃圾，而所有人都乘飞船离开了，留下机器人WALL•E收拾残局，试图把这地方变得重新适合居住起来。目前的状况或许还没那么糟糕，但毫无疑问，我们确实制造了不少垃圾。尽管回收的力度不断加大，垃圾填埋场仍逐日趋于爆满，于是乎——既然我们没有那样一个勇敢的机器人——地球上的垃圾处理专家们正致力于下一个最佳解决方案：垃圾场采矿。

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原文在这里。作者：Bijal Trivedi 译者：小菊

温室气体正从你的购物篮里释放出来。不过别担心，你能轻松地把嫌犯们揪出来。——Bijal Trivedi如是说

本地还是进口？传统还是有机？保持饮食健康的同时降低碳排放量，你能否做出这样的选择？环保化的饮食又是否真的可行？这一切真的如此重要么？

你可能会觉得惊讶，事实上，我们日常饮食所造成的温室气体排放要比开车出行高出一倍。一项最新研究证实，每个美国家庭由饮食而产生的碳排放量年均为8.1吨（CO2-eq，二氧化碳当量，一种合并了二氧化碳和其他温室气体效应的量度标准），而驾驶一辆一加仑油跑25英里的汽车（即百公里9升）开上19000公里（典型的美国人年均里程）的碳排放量约为4.4吨——显然，前者几乎是后者的两倍。

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 本文已经发表于《百科知识》，有改动

无论是“爱石油更爱淡水”的波斯湾白大袍同志们，被反恐战争和一路狂飙的石油价格轮番调戏的老美托拉斯们，忙着指腹为婚眉来眼去的老牌欧罗巴们还是和谐地与时俱进三个代表的黄皮肤们都在为一个不算新鲜的想法而激动——海水淡化。随着近几年淡水资源的进一步枯竭和海水淡化成本跳楼式的缩水，海水淡化工业已经像全球变暖一样热得让人不得不脱个光膀子大干一场。

海水淡化还是海洋浓缩？

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我们的星球上一次变暖时是什么样子

《新科学家》6.21封面文章

[作者] Anil Ananthaswamy

[翻译] 水龙吟

在读研究僧，和本科时一样主攻物理/光学方向，只不过将地点从中国平原跃迁到了美国的深山老林（还好是个滑雪胜地）。一直很喜欢科普、科幻，自己写不出来，所以从翻译开始，边学边练。

很久以前，南极洲是绿色的……而北极则有鳄鱼栖息。作者Anil Ananthaswamy指出地球上次变暖时是什么样子

 今天，即使南极最热的地方也很少有植物

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混沌初开，上帝用他那万能的双手在伊甸园布置下蔓茂的植物与乖巧的动物，又用泥巴捏出了亚当。七千年后，植物分类学之父林奈先生在人间的瑞典将“伊甸园”重现。园内阡陌纵横，从世界各地运来的植物被依照“上帝的分类法”豪华而规整地排成方阵；四条溪水流淌其间，象征了世上四大河流；整个欧洲的崇拜者前来膜拜。

然而事实证明，林奈劝说植物远没有劝说人类成功——人们接受了他的分类法，而田字格中的植物却由于不堪思乡之苦，在被禁锢的孤独中纷纷凋零。

又过了四分之一个世纪，视线转过地球几万里——

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本文发表于9月7日新京报新知周刊，有删改。

今年夏天，我国的各大水面，比去年平静。动辄几十上百米长的水怪们把名号让位于了一个叫菲尔普斯的美国小伙子；而被人当成“食人大鲶鱼”的鲸鲨，今年也没有再被大卸八块当街贱卖。（马后炮按：我这个乌鸦嘴，写完这篇，最近两天接连出现了大庆水怪和山东乳山鲸鲨。。）反而是发现“怪鱼”的报道屡见报端，广东、广西、重庆、湖北、上海、河南乃至山东，仔细看这些报道，“怪鱼”均指向了雀鳝。

名为“雀鳝“，这种怪鱼跟雀和鳝都不搭边，既没有“雀”的温存可人，也没有“鳝”的柔滑苗条，它们遍身铠甲，那身生着珐琅质的硬鳞曾经让试图一试其味的“老饕”菜刀卷刃。而且雀鳝生性凶猛，满嘴尖牙，也曾经让放生其于自己鱼塘中的大善人血本无归。

给雀鳝查查户口，就会发现这些铁甲钢牙的猛鱼并非土著，现存的7种雀鳝的原产地分布在从哥斯达黎加到南魁北克的中北美和加勒比海地区，它们多生活在淡水中，偶尔在半咸水和海水中也可以发现它们的踪迹。长久以来，雀鳝是默默无闻的，它们的肉虽然能吃，却不美味；鱼卵个头倒是很大，因为有剧毒而不能做鱼子酱。至多是被做成标本充当工艺品，或者是被渔钓爱好者当作炫技的资本。为何可以炫技？ Read the rest of this entry »

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翻译/ Tony

这是一篇投稿，投递者是Tony，blog是http://inspired5.blogbus.com/

简介这是环境科学家David Keith在2007年TED大会上发表的关于“人工气候改造”(Geo-engineering，一般译作“地球工程”)的专题演讲。他讲述了为什么这么做是可能的（甚至我们必须这么做），以及他对于这一做法的反思，即这种行为本身存在一种“道德危险”的因素。

大家可能都看过无数份关于气候变化的报道，这里我给大家看一篇刊于1953年的《纽约时报》的报道，它的内容跟你看过的此类报道大同小异。

为何我要展示这篇报道？我们事实上早在50年前已经知道有气候变化这一现象，可是直到近来有了《京都议定书》，还有许多政府要员的关注，人们才开始认真的 谈论这一问题。那我们是不是已经走在寻找解决问题的路上了？回答是否定的。过去十年里我们也在讨论气候变化的问题，可是却从没有在行动上迈出一步。

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Alexandra Witze　文　Shea　译

北极最大的冰盖当真正在不可逆转地消融吗？

当人们谈论灾难性的气候变化时，格陵兰就会浮现在大家的脑海中。如果有人将其形容为“转折点”，那是再恰如其分不过的事了。21世纪人类的命运将很大程度上取决于这个岛上的冰。如果这些冰完全融化，那么全球的海平面就会上升7米。格陵兰冰盖的消失是人类首当其冲要面对的潜在气候灾害。
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盗riset的版，装田不野的侃。环境新闻絮絮叨第一辑开讲。—————————————————-

奥运空气

奥运，北京，灰蒙蒙的天气，紧紧揪着我们和BBC记者们的心。前段时间，几个记者沿着奥运会自行车个人赛的路线，测量了七天的可吸入颗粒物的含量，结果发现大大地超标了。这个自行车个人赛的路线，从永定门到居庸关，全场200多公里。不得不说记者们很会选地方，颗粒物的污染，往往是公路工地附近最高，公园最低。正确的空气质量的测定方法，应该是全城均匀布点测量。如果用公路旁测得的数据去责怪整个北京，未免太苛刻了，还不如去直接去测汽车尾气，得到的数据会更加惊世骇俗。当然，北京有污染，环境保护部的数据也承认（http://www.sepa.gov.cn/quality/air.php3）。空气质 量的主要监控指标，包括二氧化硫，氮氧化合物，臭氧，一氧化碳和可吸入颗粒物。其中可吸入颗粒物是最让人头疼的。前四种监测指标都是气体，就像臭屁一样， 只要没人在放，那很快会消散，虽然把首钢这些放“臭气”的源头移出北京，并不是一个轻易的事情。而要消除可吸入颗粒物，那需要的不仅仅是把建筑工地移出北京，更需要把森林和绿地覆盖在北京及其周边几百公里之内所有的裸地上。这个工程，比转移首钢更加巨大，十年时间，显然是不够用。北京已经付出够多的了，接下来，请大家一起祈祷一个刮风下雨的好天气吧。

以下姑且算一个好消息：
在工地停工汽车停驶之后，北京的“臭气“污染，已经不容易被抓到把柄了。

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如今高新技术“就业”也紧张了，譬如“转基因”，也许没有别的高新技术跟它一样，在受到众多科学家推动的同时，又被无数的人所诟病了，从而导致了它的就业困难。“黄金水稻”，是“转基因作物学院”的第二期毕业生，在经过二十多年实验室的培育之后，练就一身普通水稻所不可能拥有的神奇能力之后，于2005年从植物分子育种实验室“毕业”，却至今无法“就业”。在众多科学家和社会人士的帮助之下，即使最乐观的估计，也必须等到2011年，他或许才能在亚洲的某个田间，找到自己合适的位置。

“黄金水稻”作为“转基因作物学院”毕业的“超级植物”，它拥有的特殊能力是“胡萝卜素超合成，米粒内超富集”。胡萝卜素是合成维生素A的材料，人体可以通过吸收胡萝卜素来合成维生素A，从而防止夜盲症、麻疹等疾病。世界卫生组织（WHO）估计每年有大概25万到50万的儿童因为维生素A缺 乏而失明，其中的一部分还会失去生命。他们中的绝大部分生活在发展中国家。胡萝卜，西红柿等蔬菜，以及肉，黄油和牛奶是富含维他命或者其合成材料的食物， 这些富含维生素食物可以经常在城市里寻常人家的日常餐桌上见到，但是现在不发达国家和我国农村和偏远地区的餐桌依然不可能摆上这些营养丰富的食物，2000年的国内调查表明，零到五岁的儿童体内维生素A缺乏率仍高达11％，他们大部集中在农村和偏远地区，即使在发达国家，也有像蜡笔小新一样因为不喜欢吃胡萝卜的孩子，他们很可能也缺乏维生素A。科学家们把拯救这些可怜孩子的希望，寄托在了“黄金水稻”的身上。在三年前，这个希望变成了现实。 Read the rest of this entry »

“降低二氧化碳排放，京都议定书，少吃牛肉多种树……” 这些似乎就是大部分人印象里对付全球变暖的所有对策。 但是，科学家们比想象得更富有创造力，他们正在严肃的思考一些看上去疯狂想法的可行性。这些想法之所以疯狂，就是因为他们不再被动的减少排放，减缓地球升温的速度，而是主动的出击，降低当前的温室气体在大气层中的含量。这需要勇气和疯狂。

铁豆腐渣工程

美国加利福尼亚的Planktos公司(该公司网站貌似挂了)正在进行一项鲜为人知的实验。前不久，他们的”知天鸟二号”轮船在太平洋深处倾倒了50吨的铁垃圾。这些铁屑会是”超微型浮游生物(picoplankton)”的大餐 （超微型浮游生物：从英文意思来看是皮米级的浮游生物，实际上通常指0.2 and 2 微米尺寸的浮游生物。），从而制造一起有预谋的”赤潮”。这些水中的贪吃鬼会像恶性肿瘤一样无限制的自我繁殖，覆盖大片的海面。他们通过光合作用可以吸收空气中的二氧化碳，释放氧气，从而减少温室气体在大气中的含量。 Read the rest of this entry »

原文链接：http://environment.newscientist.com/channel/earth/mg19826611.200-when-crocodiles-roamed-the-arctic.html

what it was like the last time our planet warmed up

Section: Features

Once the north pole was infested with crocodiles while Antarctica was covered by lush forests. But if the poles were so balmy, what on earth were the tropics like?

WHEN Ernest Shackleton and his men marched towards the South Pole in December 1908, they came across something entirely unexpected. After scaling the vast Beardmore glacier on the edge of the polar plateau, they found seams of coal amid the snow and ice. They also found impressions of leaves in sandstone boulders nearby and even fossilised wood from a coniferous tree.

The conclusion was extraordinary but inescapable: Antarctica was once warm and forested, conditions that could hardly be more different to the far-below-freezing midsummer weather that forced Shackleton’s team to turn back before reaching the pole. How was this possible?

Four years later, Alfred Wegener put forward his theory of continental drift which, it was later realised, could explain the balmy climate: Antarctica had been warmer because it was once much closer to the equator. Even today, some schoolchildren are taught that continental drift accounts for all the evidence for a warmer Antarctica.

However, the fossil trees Shackleton’s team discovered grew around 250 million years ago, when Antarctica was barely closer to the equator than it is today. What’s more, the continent reached its current position roughly 100 million years ago, and an ever-growing list of fossil finds date from 100 to 40 million years ago. During this time, when dinosaurs roamed the almost subtropical forests of an ice-free Antarctic, conditions on the other side of the planet were even more remarkable: the Arctic Ocean was a gigantic freshwater lake infested with crocodile-like reptiles.

As the modern world warms, there has been a surge of interest in this “hothouse” period. What sustained such high temperatures for tens of millions of years? If the poles were so warm, what were the tropics like? Recent findings provide a fascinating insight into our past - and perhaps also a glimpse of our future.

The Earth’s climate is currently in an “icehouse” phase: the polar ice sheets are not as extensive as they were during ice ages, when the sea level fell by as much as 120 metres, but some ice has remained even between ice ages. Before about 34 million years ago, though, the planet went through a prolonged hothouse phase with no ice at all. Sea level was more than 70 metres higher than today, covering vast swathes of what is now dry land. For instance, an inland ocean divided North America in two. This period lasted from the middle of the Cretaceous era until well into the Eocene - about 100 million to 50 million years ago.

One of the earliest signs that the poles were ice-free and warm 100 million years ago was the discovery at the turn of the 20th century of fossil breadfruit trees from the Cretaceous in Greenland; today such trees are at home in places like Hawaii. Since then, even more extraordinary finds have been made.

The most evocative image of a warm Arctic has emerged from the work of John Tarduno of the University of Rochester, New York. For more than a decade, Tarduno has been hunting for fossils on Axel Heiberg Island in the Canadian Arctic, just west of Greenland. The island was already well within the Arctic Circle 90 million years ago.

His team has found bones and even partial skeletons of a crocodile-like creature called a champsosaur from this period. The champsosaur was a fish-eating reptile up to 2.4 metres long that probably looked much like the gharials of India. Because these reptiles would have relied on their environment to stay warm, conditions in the far north must have been far hotter than today. “These fossils speak volumes,” says palaeoclimatologist Paul Wilson of the University of Southampton in the UK.

Last year, Tarduno’s team reported that most champsosaur remains are of juveniles, meaning the animals not only lived but bred in the Arctic. As hatchlings and juveniles could not have survived if winter temperatures came anywhere close to freezing, this means it was not only warm, but warm all year round.

Modern crocodiles are found no further north than the lower Yangtze and North Carolina. If the champsosaurs’ temperature requirements were similar, the Axel Heiberg locality must have had mean annual temperatures of at least 14 °C, and the average temperature during the coldest month could not have fallen below 5.5 °C. The region would not even have had ice in winter.

The champsosaur was not the only warmth-loving reptile to live inside the Arctic Circle. Tarduno’s team has found an abundance of fossils of four kinds of turtles at Axel Heiberg Island, again pointing to a mean annual temperature of at least 14 °C.

Most recently, the team has found fossils of a family of turtles called Macrobaenidae on Axel Heiberg Island (the details have yet to be published). These turtles originally lived in Asia, but from the late Cretaceous onwards appeared in North America too. Because turtles are very sensitive to climate, the researchers think they could have survived the migration only if they moved along a route in the far north that was warm all year round. More significantly, these turtles - like the champsosaurs - were freshwater creatures. “They would have required a non-marine connection,” says team member Donald Brinkman of the Royal Tyrrell Museum in Drumheller, Alberta, Canada. “If the Arctic was a big freshwater lake, that would have made it possible.”

Fresh water in the Arctic Ocean? As far-fetched as it seems, there is now strong evidence that as recently as 50 million years ago, at the start of the middle Eocene, at least the surface of the Arctic Ocean was fresh. This picture has emerged only recently because it is extremely hard to access the records of the ocean’s history, says Kathryn Moran of the University of Rhode Island in Kingston, a member of a 2004 expedition to drill sediment cores from the Arctic seabed.

Drill ships have to stay exactly above their chosen site to prevent the drill from snapping, yet in the Arctic drifting chunks of sea ice up to several kilometres wide make normal drilling operations impossible. “They can easily knock a ship off location,” says Moran. “So what we had to do was break that ice.” The task fell to two icebreakers. “The ships are really big and powerful, and they basically had to learn how to dance together,” says Moran.

Dance they did, and in 2004 the team collected a core of sediment that had been deposited over tens of millions of years on the Lomonosov ridge, just 250 kilometres from the North Pole. One study of the core revealed that a freshwater fern called Azolla grew abundantly in the Arctic Ocean for 800,000 years about 50 million years ago (Nature, vol 441, p 606). At the time the Arctic Ocean was largely isolated from other oceans, and fresh water from rivers would have floated on top of denser salt water. “It might have been, at least in the surface waters, one of the biggest lakes on the planet,” says Moran.

The waters of this mega-lake were a surprisingly warm 10 °C, but that’s nothing to the temperatures reached a few million years earlier during the hottest part of the Eocene, when the ocean was salty. According to another study of the core the surface water 55 million years ago was around 18 °C, peaking at an incredible 23 °C - more than warm enough for a pleasant swim at the North Pole!

What about the Antarctic? Here too gathering evidence is far from easy. Ice cores from Antarctica’s kilometres-thick ice sheets are no help, for even the oldest ice is a mere million years old. It’s the land beneath the ice that holds the secrets. “We don’t want the Antarctic ice sheet to disappear, for there is 67 metres of sea level stored there, but gosh, it would be lovely, from a palaeoclimate perspective, to know what’s under all that ice,” says Wilson. “In particular, because Antarctica has certainly been in a polar position back through the Cretaceous.”

Fossil hunters on the mainland are limited to a few exposed sites. But on the Antarctic Peninsula, a finger of land that juts north towards South America, enough rock is exposed to give explorers a glimpse not just of Antarctica’s ancient flora and fauna, but of the nature of the seas around it.

About 150 to 100 million years ago, the peninsula was a mountain range similar to the Andes, and its rivers drained into a massive basin, now called the James Ross basin. Over millions of years, the basin filled up with sediment and later the rocks it formed were uplifted. Today these rocks lie exposed on islands off the Antarctic Peninsula and contain a treasure trove of fossils from the Cretaceous, including silvery slivers of shells of ocean-dwelling ammonites and gastropods. In the late Antarctic summer, these fragments glint as they catch the sun which barely rises above the horizon. “It looks like the surface is covered in jewels,” says palaeoclimatologist Jane Francis of the University of Leeds, UK, a veteran of 12 expeditions to the poles.

Besides ammonites and gastropods, Francis and her colleagues have found abundant fossils of sea urchins and lobsters that lived on the sea floor, shark teeth, and even massive marine reptiles with rib bones about half a metre long. Oxygen isotopes in the shell fragments show that the waters around Antarctica 100 million years ago were a balmy 15 °C, compared with -2 to 0 °C today.

Dinosaur bones, which must have been washed down off the peninsula into the sea, have also been found in the marine sediments (see “Dinosaurs at the poles”). Plant fossils unearthed by Francis and her students show that 100 million years ago the peninsula was lush with ferns and cycads, along with conifers resembling the monkey puzzle tree. Analysis of the shape and size of fossil leaves has led Francis to conclude that the peninsula was very warm during the mid-Cretaceous, with a mean annual temperature of about 17 to 19 °C, similar to that of South Africa today. “That’s almost sub-tropical,” says Francis. Sweltering greenhouse Growth rings in one fossil tree trunk suggest trees thrived despite complete darkness in mid-winter. “In tree-ring terms, the tree was very happy, it wasn’t growing in any kind of stress, there’s no sign of frost rings and there’s no sign of drought,” Francis says.

Her team has also found fossil flowers dating back to about 85 million years ago. These include flowers resembling those of Siparunaceae, tropical vines found in the Amazon, as well as those of the Australian eucalyptus and Winteraceae trees such as the Tasmanian mountain pepper.

It’s abundantly clear that both the Arctic and the Antarctic were ice-free and warm from about 100 million to 40 million years ago. But until a decade ago, climate scientists struggled to explain how the Earth could have become so warm at the poles. Their models suggested it could only have happened if levels of carbon dioxide in the atmosphere were very high - turning the Earth into a sweltering greenhouse - but this would also have made the tropics extremely hot. Isotope ratios in marine shells, however, suggested that tropical waters were not much hotter than they are today.

As it turns out, the models were right and the shell studies were flawed. Recent and more careful studies by Wilson and colleagues (Geology, vol 30, p 299) suggest that tropical seas were indeed hotter during the hothouse phase, with the surface waters being as warm as 34 °C compared with 29 °C today, says Raymond Pierrehumbert of the University of Chicago, a climate researcher and contributor to the RealClimate blog.

Despite this advance, climate modellers face a new problem. While pumping up atmospheric levels of CO2 in the models creates ice-free poles and warmer tropical waters, the land in the tropics becomes unbearably hot. “The temperatures are so high that unless land plants behave differently from modern types, you would be beyond their temperature tolerance,” says Pierrehumbert. “We are talking of temperatures on land of an average of 40 °C, and with seasonal fluctuations they might even go up to 50 °C. It would kill off just about anything on land.” Today, annual mean temperatures rarely exceed 30°C.

As outlandish as these simulations seem, the models might yet again prove to be right. Researchers such as Matthew Huber of Purdue University in West Lafayette, Indiana, have only recently begun to look for evidence of plant dieback in the tropics at this time. No one had thought to look before.

There is yet another serious problem for climate modellers. The one place the models suggest did get cold during the hothouse episode is the interior of continents at high latitudes - regions like Siberia. This doesn’t fit with the evidence.

In rocks from the late Cretaceous in Siberia, Robert Spicer of the Open University in Milton Keynes in the UK and his colleagues have found plenty of evidence for ferns and flowering plants, and even possibly the pollen of palm trees (Earth and Planetary Science Letters, vol 267, p 228). Their analysis suggests that at that time Siberia’s mean annual temperature was about 13 °C, rarely touching freezing even in the winter months. “All the climate models give you very, very cold continental interiors [at high latitudes] in the winter time, so cold that you would certainly freeze palm trees and kill them off,” Pierrehumbert says.

One answer to this puzzle is to keep pumping up the CO2 levels. Models predict that the interiors of continents at high latitudes would not have frozen during the winter if CO2 levels were higher - but this means the tropics would have got even hotter.

Huber has suggested a possible answer to this dilemma: what if much more heat from the tropics was somehow carried to the poles, keeping the tropics from boiling over. He and Ryan Sriver, also at Purdue, think they have found one possible mechanism.

They studied conditions in tropical waters before and after the passage of present-day cyclones. They found that cyclones mix up the upper layers of oceans, moving heat downward. They argue that ocean currents then transport this heat towards the poles, reducing the temperature gradient between the tropics and the polar regions (Nature, vol 447, p 577). Many researchers think the intensity, frequency and duration of tropical cyclones increase with higher temperatures. If so, the amount of heat transported to the poles by cyclones would increase greatly as temperatures rise. In a hurricane-ridden hothouse Earth, this could have kept the tropics below 35 °C, while the poles simmered in subtropical heat.

However, Pierrehumbert thinks that the cyclonic heat-pump idea needs more work, and that explaining the warm interiors of continents remains a challenge. “This is now the most mysterious and toughest looking part of the problem,” he says.

Others might beg to differ. A few lines of evidence point to something seemingly impossible: ice sheets during the warmest phase of the Cretaceous. “Nobody can imagine that we had these high temperatures and at the same time we had some large glaciers in the Antarctica,” says André Bornemann of the University of Leipzig in Germany. Indeed, models cannot replicate these conditions.

One recent study by Bornemann’s team suggests that for a 200,000-year period around 91 million years ago, there were ice sheets at least half the size of the ones that blanket Antarctica today. The evidence comes from oxygen isotope ratios in shells from the Atlantic seabed (Science, vol 319, p 189).

However, a similar study by Wilson’s team found no evidence of glaciation (Geology, vol 35, p 615), so this issue is far from settled. But if ice sheets can grow suddenly even during hothouse periods, Wilson point out, it means the climate can change more suddenly and dramatically than anyone thought. “That really demands being understood.”

Despite these vexing issues, there is a growing consensus that the hothouse climates were due to high levels of CO2 in the atmosphere. But where did it come from?

Among other things, the amount of CO2 in the atmosphere depends on the balance between volcanic activity and the weathering of rocks. High volcanic activity during the Cretaceous might have kept the level of CO2 high, says Wilson. Later on, volcanic activity may have fallen and weathering increased as the Himalayas began to form, pushing Earth into an icehouse phase.

However, while CO2 levels up to a million years ago can be directly measured from, it’s much harder working out what they were 100 million years ago. Researchers have to rely on proxies such as the number of pores in fossil leaves, and. Pinning down these numbers is critical, for this would tell us just how sensitive the climate is to rises in CO2.

Some models suggest CO2 levels were 16 times as high as pre-industrial levels during the Cretaceous and Eocene hothouses, while others suggest eight times. Despite the uncertainties, eight times fits in far better with the proxy data, suggesting that the climate is highly sensitive to rises in CO2.

This does not bode well for us, given the amounts of CO2 we are dumping into the atmosphere. CO2 levels look set to double from pre-industrial levels and if we keep failing to curb emissions, they could quadruple within 200 years. “Then we are half way towards the CO2 levels that turned the world into the Cretaceous hothouse,” says Pierrehumbert.