This Wednesday is Valentine’s day. Chances are high your thoughts are already floating off to your date night. Although it might seem as if there are few things further apart than love and climate change, I figured out they can affect each other in (sometimes scarily) significant ways.

You might not give a damn about rising seas and melting permafrost, but beware: hotter temperatures might affect action in the bedroom. What a bummer! In a study from the National Bureau of Economic Research in the US a couple years back, three researchers found indications that exceptionally hot days lead to a reduction in birth rates nine months later. To put it in the authors own words: “Extreme heat could raise the physiological cost of coitus on a given fertile day, leading to a shift in coital frequency to some subsequent day(s).” In layman’s words: when it’s too hot to bang, people usually catch up later. But postponing the sex doesn’t make up entirely for the decline, as the study of US birth rates between 1931 and 2010 suggests.

One should interpret the study with a grain of salt though. There are enough hot countries where there is no shortage of babies – quite the opposite. It’s not so much the absolute temperature that keeps people from diving under the blankets, rather an extreme temperature spike. Many other factors play a role in the number of times people make love. For many people, holidays are the favorite moment — no matter if it’s a hot or a cold country.

On top, the lower birth rates are probably not only the result of a ‘reduction in coital frequency’. It has been shown that higher temperatures also lead to lower testosterone levels and poorer semen quality in men, while women’s menstruation and ovulation can be distorted. Those effects might also be to blame for the reduction in successful impregnations.

By default, slightly more boys than girls are born. Yet, a team of scientists that studied the effect of the extremely hot summer of 2010 and exceptionally cold winter of 2011 in Japan, came to the conclusion that the extremes reduced the ration of male versus female babies. That’s correct: climate change seems to be closing the gap between male and female newborns. This is due to the fact that male fetuses are more vulnerable to external stress factors and hence have a higher fetal rate in periods of extreme temperatures.

All in all, it’s quite hard to draw concrete conclusions on the effects of climate change on human reproduction, due to the limited availability of scientific literature on the topic. On the other hand, the effects on ‘the birds and the bees’ have been studied a lot. For example, it has been proven that nearly 400 UK plant species were flowering on average 4.5 days earlier than a decade before. The concern here is that plants and their pollinators might not adapt to changes in the seasons at the same pace.

For sea turtles, rising temperatures cause a sex-bias towards more female hatchlings. Although on the short term that could mean that the remaining male turtles just get lucky more often, a temperature rise above 3 degrees Celsius could lead to the extinction of the turtle population because there are not enough males to sustain it.

For us humans, things will not get out of hand that quickly. Yet already now climate change induced extreme weather events have a perverse side effect that is more worrisome than somewhat less frequent sex or lower testosterone levels in men. In the poorest countries in Africa and Asia, child marriages are on the rise again. The reason is simple: with falling crop yields, parents can’t feed all their kids anymore or pay the school fees. Marrying off one of the girls in the family means one less mouth to feed. Sometimes the girls decide to leave themselves, hoping that their husband can support them to keep going to school.

Not all child marriages happen due to climate change-induced poverty, but in an African country like Malawi, it is estimated to be responsible for about 30-40% of the 5 million girls at risk of being married before they reach the age of 18. Those numbers are very scary.

Meanwhile, in the developed world women are giving birth to fewer and fewer kids, to the extent governments are desperately looking for ways to get people to make more babies. In Denmark, travel agency Spies Rejser decided to take matters in its own hands and save the country’s future by giving couples an ‘ovulation-discount’. Danes have 46% more sex on holidays than when at home, you know. If couples provided proof they made a baby during their trip, they could even win three years of baby supplies.

I hear you thinking: ‘Isn’t overpopulation one of the underlying problems of the climate crisis?’. Indeed, 7 billion people are using up Earth’s resources faster than it can replenish. Yet, the developed countries with low birth rates are also among the richest. We need them at the front line of the fight against climate change, instead of worrying how to support their aging population. Meanwhile, everything has to be done to reduce a rich person’s carbon footprint.

Although fewer in numbers, babies become more and more international. In Europe, the now 30-year old Erasmus study exchange program sparked cross-border romances that resulted in an estimated one million ‘Erasmus babies‘. I am not going to say here that international love is a no-go for the planet due to increased travel emissions, but if you need to travel far to see your lover this Valentine’s day, try to use a train or bus. If that takes very long, meet in the middle (isn’t that a romantic idea?).

And whatever you plan to do on your date, keep it low key and don’t buy tonnes of stuff for your loved one. Why not go for stroll through the park at sunset? A vegetarian dinner? Plenty of eco-friendly options! In any case, enjoy your Valentine’s day and don’t forget that there are 100% natural condoms these days 😉.