近200万年前地球的二氧化碳循环

古老的冰揭示了近200万年前地球的二氧化碳循环

After more than two years of analysis, the oldest complete ice core has served up a reliable snapshot of our planet's atmosphere from nearly 2 million years ago. And the results are not what many expected.

经过两年多的分析，最古老的完整的冰核提供了近200万年前地球大气的可靠快照。结果也不是很多人所期望的。

We know that roughly 1 million years ago, the cycle of Earth's ice ages suddenly shifted, with deeper and longer freezes occurring only every 100,000 years, instead of once every 40,000 years.

我们知道，大约100万年前，地球冰期的周期突然发生了变化，每10万年才会发生一次更深、更长的冰期，而不是每4万年一次。

Nothing in our planet's orbit could explain the 'abrupt' change, known as the Mid-Pleistocene transition (MPT), and with few other explanations, some hypothesised there must have been a long-term decline in atmospheric CO2 concentrations, cooling the planet to a new threshold.

在我们星球的轨道上，没有任何东西可以解释这种被称为中更新世转变(MPT)的“突然”变化，而且由于几乎没有其他的解释，一些人猜测一定是大气中的二氧化碳浓度长期下降，使地球降温到一个新的阈值。

Ancient air bubbles trapped in the Antarctic ice sheet have now revealed a somewhat different picture. Dating back roughly 1.5 million years, these tiny doses of our ancient atmosphere contain "amazingly low" CO2 levels, according to paleoclimatologist Yige Zhang from Texas A&M University, who was not involved in the study and who told Science Magazine he found the results "quite interesting".

被困在南极冰盖的古代气泡现在呈现出了一幅有些不同的画面。来自德克萨斯农工大学的古气候学家张翼格(Yige Zhang)表示，这些微小的古大气中二氧化碳含量“低得惊人”，这可以追溯到大约150万年前。张翼格没有参与这项研究，他对《科学》杂志(Science)说，他发现这个结果“非常有趣”。

These are the first direct observations of atmospheric greenhouse gases before the ice ages on Earth began to grow longer, and they suggest something other than a long-term decline in CO2 was at play to shift our planet's entire ice age cycle.

这些是在地球冰河时代开始变长之前，对大气温室气体的首次直接观测，它们表明，在改变地球整个冰河时代周期的过程中，除了二氧化碳的长期下降外，还有其他因素在起作用。

The oldest sample of ice we could test for CO2 levels previous to this new core was a 'mere' 800,000 years old; other estimates based on the chemistry of Earth's sediments are only useful as a proxy for greenhouse gas levels - they're not useless, but more verification is necessary.

在新地核形成之前，我们能够检测到的最古老的冰样本的二氧化碳含量只有80万年;其他基于地球沉积物化学成分的估算只能作为衡量温室气体水平的指标——它们并非毫无用处，但需要更多的验证。

The new ice analysis, which used more precise measurements than before, has revealed that "although CO2 levels during glacials stayed well above the lows that occurred during the deep glacials of the past 800,000 years, the maximum CO2 concentrations during interglacials did not decline," Earth scientist Eric Wolff from the University of Cambridge writes in a review of the research.

新的冰分析,使用比以前更精确的测量,揭示,“虽然在冰川的二氧化碳水平仍远远高于低位的深冰川发生在过去的800000年里,最大的二氧化碳浓度在间冰期没有下降,”剑桥大学的地球科学家埃里克·沃尔夫在审查的研究中写道。

One of the important results of this study is to show that carbon dioxide is linked to temperature in this earlier time period, says atmospheric scientist Ed Brook from Oregon State University.

“这项研究的一个重要结果是表明二氧化碳与早期的温度有关，”俄勒冈州立大学的大气科学家Ed Brook说。

That's an important baseline for understanding climate science and calibrating models that predict future change.

“这是理解气候科学和校准预测未来变化的模型的重要基础。”

In other words, the relationship between CO2 levels and Antarctic temperature hasn't changed much over this time span. And the lower CO2 levels during ice ages are probably just a consequence of the shorter glacial periods that occurred before the MPT.

换句话说，在这段时间内，二氧化碳水平和南极温度之间的关系并没有改变多少。在冰河时代，二氧化碳含量较低可能只是MPT之前较短的冰河时期的结果。

In fact, the authors found the lowest levels of CO2 didn't show up for the first 40,000 years after the MPT occurred.

事实上，作者发现，在MPT发生后的最初4万年里，二氧化碳的最低水平并没有出现。

Our findings appear to be inconsistent with hypotheses that attribute the transition into the 100k world to a long-term decline in both interglacial and glacial atmospheric CO2, they write.

他们写道:“我们的发现似乎与那些将向10万千瓦世界的转变归因于间冰期和冰川期大气二氧化碳长期减少的假设不一致。”

In his review of the research, Wolff commended the researchers on their precise estimates and "tantalising" snapshots. But he argues a more "complete, undisturbed time series" is needed to put CO2 levels in context.

在他的研究综述中，沃尔夫称赞了研究人员的精确估计和“诱人的”快照。但他认为，需要一个更“完整、不受干扰的时间序列”来解释二氧化碳水平。

Luckily for us, the ancient ice core, which was found in Antarctica's Allan Hills, may soon have company. Researchers say the ice sheet here dates back 2.7 million years and maybe even longer.

对我们来说幸运的是，在南极洲的艾伦山发现的古老的冰核可能很快就有同伴了。研究人员表示，这里的冰原可以追溯到270万年前，甚至更早。

Given how much the ice has shifted in this region, any new cores we dig up will probably come in discontinuous sections, but their information may help us piece together at least some of the secrets of our planet's mysterious past.

考虑到这一地区的冰已经发生了巨大的变化，我们挖掘出的任何新核心可能都是不连续的部分，但它们的信息可能会帮助我们拼凑起至少一部分地球神秘过去的秘密。

We don't know the age limit in this area, Brook says. "It could be much older in some places. That's why we're going back. Pushing beyond two million years would be pretty amazing."

“我们不知道这个地区的年龄限制，”布鲁克说。“有些地方可能更古老。这就是为什么我们要回去。如果能超过200万年，那就太神奇了。”

The study was published in Nature.

这项研究发表在《自然》杂志上。