In a collection of essays published posthumously on Tuesday, Hawking wrote that climate change and the possibility of nuclear war are putting humans in grave danger, adding that the latter is likely the biggest threat to humanity. The scientist, who died in March, wrote in Brief Answers to the Big Questions that people treat the Earth with "reckless indifference," which could result in our own extinction if we don't find another home.
"One way or another, I regard it as almost inevitable that either a nuclear confrontation or environmental catastrophe will cripple the Earth at some point in the next 1,000 years," Hawking wrote.
Hawking said it was hard for him to maintain his optimism in light of all the problems in the world, particularly political instability and global warming. The population is becoming too large for this planet, and physical resources are quickly running out, he pointed out. A number of issues - including deforestation, the extinction of animal species, rising temperatures, and lack of water - can be solved, though we are failing to take appropriate action, Hawking wrote.
He said scientists have a duty to inform the public about the dangers we face, even as politicians who deny the existence of man-made climate change ignore these warnings. If it hasn't already, global warming may become self-sustaining soon, Hawking wrote. For example, the melting of ice caps in the Arctic and Antarctic could lower the amount of solar energy that reflects back into space, which would then brings the global temperature up even more.
The Earth could end up looking a lot like Venus, Hawking said, with sulphuric acid rains and a boiling temperature of 250 degrees Celsius.
In addition, a catastrophic asteroid collision is inevitable, Hawking wrote. (The last big impact, believed to have wiped out dinosaurs, occurred 66 million years.) As a more immediate threat, Hawking said he is concerned about a possible nuclear war, saying there are enough nuclear weapons to destroy humans multiple times over.
Even if we escape to another planet, it may not be possible to save millions of other species. Their extinction, Hawking said, "will be on our conscience as a race."
Though we have nowhere else to go right now, Hawking said humans are explorers and need to channel their curiosity to find new homes. Scientists have proven the Earth is not flat and brought people to the Moon; now, it is time to explore new solar systems, he wrote.
"We need to rekindle the excitement of the early days of space travel in the 1960s," Hawking wrote. "Spreading out may be the only thing that saves us from ourselves."
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