地球历史上大气中氧气含量的变化影响全球气候

中国科技网6月12日报道（张微 编译）在地球的整个历史过程中，大气层中氧气含量的变化显著改变了全球气候。新密歇根大学领导的一项新研究得出的结论是：重建过去气候情况的努力必须包含这个之前被我们忽略的因素。

目前，地球大气层中氧气的含量是21%，但在过去的5.41亿年里，这个含量是在10%到35%之间变动的。

根据密歇根大学的古气象学家克里斯托弗·保尔森的研究，在氧气含量下降期间，由此而来的大气密度下降造成了地表水分蒸发的加剧，这反过来又导致降水的增加和气温的升高。

“氧气含量和气候之间的联系从未被考虑。而这种关联性是地质时段的一个重要因素，”地球与环境科学系教授保尔森说。虽然不像导致温室效应的二氧化碳气体浓度对气候变化那么重要，但氧气也发挥着关键作用，他说。

“不被认为是二氧化碳浓度的变化造成的，氧气浓度可以帮助解释古气候记录的特征，如果我们想要全面了解过去的气候情况，那么氧气含量是必须要考虑的因素，”保尔森说。“但是，氧气含量的变化在当今气候变化中并不是一个重要的影响因子。”

这样研究成果计划发表在6月11日《科学》杂志网络版上。

在整个地球的历史上，氧气含量不断上升和下降。除去氧分子就会使大气层变得稀薄，增加了阳光直射地表的可能。

更多的光照意味着地表水分蒸发的加剧，这导致了更高的湿度和更多的降水。当湿度水平上升时，温度也增加，因为水蒸气是一种强效的热传递“温室”气体。

增加氧分子具有相反的效果：较厚的大气层，分散的阳光辐射，降低的地表蒸发量，较少的水蒸气热传递。

在他们发表在《科学》杂志的论文中，保尔森和他的两个同事量化了氧气浓度的变化对气候的影响，利用一个大气的全球气候模型来解释大气密度，质量和分子量的变化。

研究团队计算机模拟重点关注白垩纪中期，这个时期的特征是大气中二氧化碳含量较高，也是过去1亿年来最热的时期。具体来说，他们关注的是1.005亿年到9390万年之间的塞诺曼期。

他们根据氧气水平的变化进行了一系列的模拟研究，从氧气含量较低的5%到最高的35%。研究团队发现氧气水平下降导致全球降水量和温度的大幅度提高。

研究人员总结道，氧气浓度的变化有助于解释古气候记录的特征，不被认为是二氧化碳浓度的变化造成的，如温暖的极地气候以及一些历史时期出人意料的高降水率。

虽然之前没有考虑氧气含量对气候的影响，大气中氧气含量的变化一直被公认为塑造了地球上生命的进程。例如，几十亿年前，海洋中光合作用的蓝藻释放出大量的氧气，最终使得动物在这个陆地上的生存成为可能。

英文原文：

Variations in atmospheric oxygen levels shaped Earth's climate through the ages

Variations in the amount of oxygen in Earth's atmosphere significantly altered global climate throughout the planet's history. Efforts to reconstruct past climates must include this previously overlooked factor, a new University of Michigan-led study concludes.

Oxygen currently comprises about 21 percent of Earth's atmosphere by volume but has varied between 10 percent and 35 percent over the past 541 million years.

In periods when oxygen levels declined, the resulting drop in atmospheric density led to increased surface evaporation, which in turn led to precipitation increases and warmer temperatures, according to University of Michigan paleoclimatologist Christopher Poulsen.

"The connection between oxygen levels and climate has never been considered. It turns out that it's an important factor over geological timescales," said Poulsen, a professor in the Department of Earth and Environmental Sciences. While not as critical to climate as levels of heat-trapping carbon dioxide gas, oxygen plays a key role, he said.

"Oxygen concentration can help explain features in the paleoclimate record not accounted for by variations in carbon dioxide levels, and it must considered if we are to fully understand past climates," Poulsen said. "However, variations in oxygen levels are not an important factor in present-day climate change."

The study is scheduled for online publication in the journal Science on June 11.

Throughout Earth's history, oxygen levels repeatedly rose and fell. Removing oxygen molecules thins the atmosphere, increasing the likelihood that incoming sunlight will make it to the surface without getting scattered away.

More sunlight means more evaporation from the surface, which leads to higher humidity levels and increased precipitation. As humidity levels rise, temperatures also increase because water vapor is a potent heat-trapping "greenhouse" gas.

Adding oxygen molecules has the opposite effect: a thicker atmosphere, more scattering o incoming sunlight, reduced surface evaporation, and less heat trapped by water vapor.

In their Science paper, Poulsen and two colleagues quantify the effect of changing oxygen levels on climate using an atmospheric global climate model to account for changes in atmospheric density, mass and molecular weights.

The team's computer simulations focused on the mid-Cretaceous, a period characterized by high atmospheric carbon dioxide levels and the warmest conditions of the last 100 million years. Specifically, they focused on Cenomanian Age, from 100.5 million years ago to 93.9 million years ago.

They developed a series of simulations in which oxygen levels varied from a low of 5 percent to a high of 35 percent. They found that decreased oxygen levels led to substantial increases in global precipitation rates and temperature.

Changing oxygen concentrations could help explain features of the paleoclimate record not accounted for by variations in carbon dioxide levels, such as warm polar temperatures and unexpectedly high precipitation rates in some periods, the researchers conclude.

Though previously unappreciated for its influence on climate, changing atmospheric oxygen levels have long been recognized for shaping the course of life on Earth. Billions of years ago, for example, photosynthesizing cyanobacteria in the oceans released massive amounts of oxygen that eventually made it possible for animals to colonize the land.