Rainforest Response to Pennsylvanian Global Warming

如果全球变暖继续下去，对我们的星球会有什么影响？更具体地说，地球上最丰富的生态系统——热带雨林——将如何应对未来的变化？这些问题的一些答案可能存在于古代的地质历史中。三亿年前，地球经历了大规模的全球变暖，极地冰盖完全融化。大约在同一时间，热带雨林开始枯死。全球变暖引发了物种灭绝吗？并非所有人都同意。另一些人则把矛头指向地球构造板块的挤压，它迫使热带边缘的喜马拉雅山规模的山带上升。他们声称海拔和地貌的变化导致了雨林的灭绝。从2008年到2013年，我计划带领一个庞大的国际专家团队，对这一系列重要事件进行详尽的调查。我们想知道雨林枯死是如何以及为什么发生的。这是一个缓慢而持久的衰退还是突然的紧缩，是全球变暖还是造山？为了实现这一目标，我们打算在美国伊利诺斯州、肯塔基州和印第安纳州的地下煤矿开展工作。这里保存着从关键时期开始的壮观的森林化石。今年4月，当我们首次报道这些惊人的发现时，它们真的抓住了公众的想象力，引发了“媒体狂潮”。这些森林化石的非凡之处在于，它们的出土面积达数千公顷（相当于一座城市的大小）。这使得像我这样的地质学家可以真正穿过森林，看看物种构成在古代景观中是如何变化的。这向我们展示了森林是非常复杂的，斑块状的生态系统，就像今天的雨林一样。我的团队现在已经确定了七个相互叠加的森林化石。这些森林化石跨越了大约200万年的时间，关键是在雨林枯死之前和之后。通过研究这些森林，我们可以确切地了解枯死是如何发生的。哪些物种灭绝了？生态系统是如何重组的？这种变化有多突然？当然，以这种细节了解雨林枯死并不能告诉我们原因。然而，我们的北美雨林距离最近的山带有一千多公里，所以我们可以排除这种变化的原因。那么全球变暖呢？为了找出灭绝事件期间的气候变化，我们计划研究化石树叶子上的气孔。气孔允许植物吸收二氧化碳。研究表明，空气中的二氧化碳越多，植物产生的气孔就越少。所以，实际上，树叶化石可以让我们探测到二氧化碳水平的变化。由于二氧化碳是一种重要的温室气体，其浓度的变化会影响气候。我的同事们将使用其他技术，利用化石土壤来独立估计过去的二氧化碳水平。我们将一起在关键的时间间隔内整理出高分辨率的气候变化记录。然后我们将看到气候变化的时间是如何与雨林枯死的时间相对应的。当然，即使事实证明这两个事件同时发生，也不一定意味着气候变化导致了雨林枯死。你永远无法在化石记录中证明“因果关系”。然而，同事们将把我们的发现输入到计算机气候模拟中。这将测试我们观察到的气候变化是否会导致观察到的雨林枯死。这将有助于支持我们对气候变化和物种灭绝的推论。虽然我们的三亿年前的热带雨林不能直接与现代热带雨林相比，但我们的工作将普遍有助于讨论面对气候变化的热带雨林枯死的性质和速度，并帮助我们更好地预测未来的变化。

If global warming runs its course, what will be the effects on our planet? More specifically, how will the Earth's richest ecosystems - the tropical rainforests - cope with future changes? Some of the answers to these questions may lie in the ancient geological past. Three hundred million years, the Earth experienced global warming on a massive scale, completely melting the polar icecaps. About the same time, the tropical rainforests began to dieback. Did global warming trigger extinction? Not everyone agrees. Others point the finger at the jostling of the Earth's tectonic plates that forced up a Himalayan-scale mountain belt on the edge of the tropics. They claim that changes in elevation and geomorphology caused rainforest wipeout. From 2008-2013, I plan to lead a large team of international experts to investigate this important sequence of events in enormous detail. We want to know exactly how and why rainforest dieback occurred. Was it a slow protracted decline or an abrupt crunch, and was global warming or mountain building responsible? To achieve this end, we intend to work in the underground coalmines of Illinois, Kentucky and Indiana, USA. Here spectacular fossilized forests are preserved that date from the crucial time period. When we first reported these amazing discoveries back in April, they really captured the public imagination and triggered something of a 'media frenzy'. The extraordinary thing about these fossil forests is that they have been unearthed over thousands of hectares (the size of a city). This allows geologists like me to literally walk through the forests and see how species make-up changed across the ancient landscape. This shows us the forests were very complex, patchy ecosystems much like rainforest today. My team has now identified seven fossil forests stacked on top of each other. The fossil forests span a period of about two million years and crucially date from before and after the rainforest dieback. By studying these forests we can learn exactly how dieback occurred. What species were lost? How did ecosystems reorganize? How abrupt was the change? Of course, understanding rainforest dieback in this kind of detail doesn't tell us the cause. However, our North America rainforests were located more than a thousand kilometers from the nearest mountain belt, so we can rule out that option that a cause of change. So what about global warming? To find out what the climate was doing during the extinction event we plan to study pores in the leaves of the fossil trees called to stomata. Stomata allow plants to absorb carbon dioxide. Studies have shown that the more carbon dioxide there is the air, the less stomata plants produce. So, in effect, fossil leaves allow us to detect changes in the carbon dioxide levels. As carbon dioxide is an important greenhouse gas, changes in its concentration will affect climate. My colleagues will use other techniques using fossil soils to get independent estimates of past carbon dioxide levels. Together we will put together a high-resolution record of climate change through the key time interval. We will then see how the timing of climate change corresponds with the timing of rainforest dieback. Of course, even if it turns out that two events coincide, it doesn't necessarily mean that climate change caused rainforest dieback. You can never prove 'cause and effect' in the fossil record. However, colleagues will input our findings into computer climate simulations. This will test to see whether our observed climate change would lead to the observed rainforest dieback. This will help support any inferences we make about climate change and extinction. Although our three hundred million year old rainforests are not directly comparable to modern rainforests, our work will generally contribute to discussion on the nature and rate of rainforest dieback in the face of climate change, and help us better predict future changes.