Last night's episode of Cosmos was about carbon dioxide, the greenhouse effect, and the hopeful possibilities for humanity's future on Earth.

Cosmos Knows It's Not Easy Being Green

We begin our journey with our nearest planetary neighbor, Venus, with its famously harsh environment. Venus lost her oceans long ago, and with them her ability to sequester carbon dioxide released by geological processes. The increased temperature and pressure lead to a positive feedback loop, with the release of more carbon dioxide warming the planet and leading to the release of more carbon dioxide. Meanwhile, back here on Earth, algae in the sea sequesters carbon dioxide, maintaining our climate on the razor's edge between ice age and global heat wave.

It is this razor's edge on which humanity is threatening to cut itself. Tyson describes the data obtained from ice core samples showing that Earth has never had more than 300 parts per million of CO2 at any time over the last 800,000 years...until now. We've just peaked over 400 parts per million. Since the beginning of the industrial revolution, atmospheric CO2 has increased by 40%.

Carbon dioxide in the atmosphere retains heat. Without any CO2, the Earth would be a frozen ball of ice. "There is nothing controversial about this," says Tyson. "It's just basic bookkeeping of the energy." It doesn't matter that your town had a cold winter last year. What matters is that, all over the world, the temperature of the atmosphere and ocean is increasing as a general trend.

Cosmos Knows It's Not Easy Being Green

But how sure are we that the increased CO2 in the atmosphere is industrial and not natural? Could it be volcanic? No. The isotopic signature of the CO2 in the atmosphere matches fossil fuels, not volcanic eruptions. What if it's the sun? Could increased solar output be warming the ocean and atmosphere? No, the Earth is warming more at night and in winter and observations indicate that solar output has been stable for decades. The warming we are seeing is consistent with an increase in greenhouse gases, not an increase in solar radiation.

While there is disagreement on the specifics, the case for human contribution to global warming is as conclusive as theories in the natural sciences get. And it's no Johnny-Come-Lately hypothesis. Going back as far as 1896 with Svante Arrehenius' first theoretical models of global warming, through 20th century scientists E O Hulburt, Guy Callender and Carl Sagan, the sensitivity of our climate to even modest changes in CO2 concentration has long been a subject of study.

We feed our people, fight disease and engineer entire cities on the basis of science as well established as the greenhouse effect. And few can doubt that if manmade climate change was found to be beneficial, those who deny the evidence today would have little trouble accepting it tomorrow. It is not the science of climate change that we resist, rather the unpleasant set of choices we imagine we will face in dealing with it.

But climate science need not be reduced to a simple blame game or a mixed bag of unattractive options for our future. The same solar dynamo that created all the fossil fuels we're so eager to burn also drives the winds and warms the surface of the Earth. Ultimately, fossil fuels are just a form of solar energy. Even if they were perfectly harmless to the environment, we'd still have plenty of reason to pursue green energy, just for efficiency's sake.

Tyson ends the episode with a paen to the Apollo program and the invention of agriculture and a call for us to become the latest, greatest generation to advance the state of human achievement. In confronting the reality of global climate change, we are presented with clear options: we can stick our heads in the sand, pretend that science isn't science and forbid ourselves from even studying the issue, hoping against hope that the scientists who dedicate their lives to the study of the climate are all liars or fools, or we can rise to the challenge of building a world worth inheriting not, as President Kennedy said, because it is easy, but because it is hard.

It's time to decide.