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How we can transform our energy system to achieve net-zero emissions

Killing fossil fuels to halt global warming is the greatest challenge we face. We now have a masterplan of what we must do when - and there's no time to delay

TURN on the nearest switch. You won't notice anything different; that is kind of the point. Yet in many places, there is a better chance than ever that the electricity coming out of the socket was generated by clean, renewable sources such as solar panels and wind turbines.

That is progress, of a sort. In most countries, however, most electricity still comes from climate-polluting, fossil-fuel sources. Your heating, too, almost undoubtedly uses fossil fuels, as does your car, if you have one. Most goods you buy require fossil fuels to make them and transport them to the shop or to your front door. And if this is the world you live in, you are a lucky one: access to affordable, reliable, convenient energy of any sort is far from a given in many parts of the globe.

That is the background for an energy revolution that needs to happen over the next three decades if we are to hit net-zero carbon emissions, and limit global warming to a 'safe' 1.5°C. 'The scale and speed of the efforts demanded by this critical and formidable goal make this perhaps the greatest challenge humankind has ever faced,' said Fatih Birol, the head of the International Energy Agency (IEA), in May, as he unveiled the agency's landmark report Net Zero By 2050.

That report contained few surprises about what we need to do. The big two questions remaining are whether we will actually do it, and what sort of world we end up making in the process.

Scientists have worked out how to send viable mouse sperm on postcards

A simple way to transfer sperm in the mail could find an application in scientific studies and animal husbandry.

Sperm is often transported nationally and internationally, but the glass vials typically used for transport are vulnerable to breaking in transit.

A team led by Daiyu Ito at the University of Yamanashi in Japan has now come up with a way to deliver sperm that removes the risk of sample loss. The method involves placing sperm on a sheet of paper and popping it in the post.

'Until now, sending mouse sperm to other researchers has required a freezing environment such as liquid nitrogen or a freezer,' says Ito. 'Not only is the constant supply of liquid nitrogen and electricity needed for transportation expensive to maintain, but if there is a road disruption or power outage due to an earthquake, all the sperm will melt and become unavailable.'

The researchers thought that an alternative might be to freeze-dry sperm onto a surface and send it through the standard mail. After testing several materials, including filter paper and vinyl sheets, the team discovered that weighing paper - a form of paper used to hold samples being weighed in scientific analyses - was the best option. Mouse sperm could be freeze-dried onto the paper and still remain viable when it was retrieved later. Thousands of mouse sperm samples could be stored this way in a single book, which the researchers have called a 'sperm book'.

To put the method to the test, the researchers freeze-dried mouse sperm to a sheet of weighing paper, before sandwiching the paper between plastic sheets for easier handling and sending it through the Japanese postal service, either in an envelope or attached to a postcard.

When the samples were retrieved after travelling 200 kilometres - from the University of Tokyo to the University of Yamanashi - over the course of two days, the researchers could use the sperm to produce healthy mice.

The simplicity of this delivery method means that there is potential for misuse, including the illegal transport of genetic material. This means new regulations will need to be considered, says Ito - whose team previously sent mouse sperm into space.

It is also perhaps unlikely that the new method will be used in human fertility treatments, says Ryosuke Kaneko at Osaka University in Japan, who wasn't involved in the analysis.

'Given the potential for human assisted reproductive technologies to increase the risk of some diseases in the later years of offspring, caution should be exercised in applying the present results to the preservation of human sperm,' says Kaneko. 'However, this study raises the intriguing idea that human spermatozoa could be preserved and transported by less cost, more secure, space-saving and simple methods.'