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A jet takes flight from Sky Harbor International Airport as the sun sets over Phoenix, Az., on July 12, 2023.Matt York/The Associated Press

Clayton Page Aldern is a senior data reporter at Grist and a research affiliate at the Center for Studies in Demography and Ecology at the University of Washington. He is the author of the new book The Weight of Nature: How a Changing Climate Changes Our Brains.

The year is 2055 and you are on trial for murder. The unfortunate fact is: You killed a man. Put a gun against his head, pulled the trigger – didn’t even have time to write a song about it before you were detained. Also: It was 43 C outside.

The temperature is relevant to your murder trial, because your lawyer, a rising star in the legal scene with a penchant for innovation, has submitted a not-guilty plea on your behalf by reason of insanity. In particular, the defence has claimed something called temporary thermogenic delirium.

This particular plea has become somewhat fashionable in the activist legal circles of the 2050s. It says: Extreme heat has a reliable and well-characterized slate of effects on the human brain, and in particular, it primes its owner toward impulsivity, retaliatory behaviour and aggression. There’s a non-negligible chance that you wouldn’t have pulled this particular trigger on a more temperate day. So can a jury reasonably assign guilt in this instance? Is it fair to accept the usual premise that you acted of your own accord? Your lawyer doesn’t think so.

The hypothetical isn’t so far-fetched. The interplay between heat and violence is a relationship people have understood – at least on an anecdotal and intuitive level – for thousands of years. You understand it, too. Certainly you have felt more aggressive (quicker to the trigger, as it were) on a hotter day. As the world warms, it follows, people are more likely to feel and act on these impulses. We’ve already begun to take note of these effects in society. Postal workers file approximately 5 per cent more discrimination and harassment complaints on workdays above 32 degrees, relative to days with highs between 15.5 and 21 degrees. Baseball pitchers intentionally hit more batters at higher temperatures. Drivers are quicker to honk their horns. Domestic violence spikes after a heat wave.

Psychologists and behavioural economists have sought to understand exactly how many additional violent acts the world is set to witness under climate change. For example, Matthew Ranson, a Harvard environmental economist, published in 2014 a paper estimating that, given empirical data on the relationship between heat and violence, global warming will increase the rates of murder and aggravated assault in the United States by more than 2 per cent. Perhaps that figure doesn’t sound particularly extreme. To place it in absolute terms: “Between 2010 and 2099,” he wrote, “climate change will cause an additional 22,000 murders, 180,000 cases of rape, 1.2 million aggravated assaults, 2.3 million simple assaults, 260,000 robberies, 1.3 million burglaries, 2.2 million cases of larceny, and 580,000 cases of vehicle theft in the United States.”

Dr. Ranson’s use of “cause” here is notable. It acknowledges the manners in which environmental change bears directly on behaviour – how it infringes on our perceived agency.

Over the past four decades or so, neuroscience and psychology have confirmed the physical and predictable bases of these relationships. In the case of heat and aggression, warmer temperatures appear to prime people toward impulsivity and a tendency to interpret otherwise benign interactions – such as being bumped into or hearing a joke made at your expense – as aggressive. Heat disturbs the functional connectivity between brain regions necessary for executive control and increases connectivity amongst the organ’s limbic system, a series of brain structures (such as the amygdala) responsible for representing and regulating our emotions. And as Finnish researchers showed in 2017, extreme heat also acts on the brain’s serotonin network in a manner that appears to modulate violence, with the correlation between heat and blood levels of a protein necessary for serotonin transmission most tightly coupled in Finns who had committed violent crimes. Levels of the protein in this cohort could explain nearly 40 per cent of Finland’s violent-crime rate over an 18-year period.

That researchers have come so far as to trace the influence of weather through to its neurochemical and behavioural consequences should give us pause as we consider the manners in which decisions are assigned any flavour of moral value. Your hypothetical lawyer, informed by the neuroscience of the future, may indeed have access to even further proof illustrating the mechanisms by which environmental influence saps us of anything we could reasonably call free will. And without free will, doesn’t legal culpability go out the window, too?

It’s an age-old question thrown into stark relief by a warming world – not least because the heat–aggression link is but one vector through which a changing climate challenges the assumption of free will.

Consider, too, the manners in which heat acts on our abilities to evaluate data and make informed decisions accordingly. To pluck just one example off the shelf: U.S. immigration judges are less likely to hand down decisions favourable to asylum seekers on hotter days. In particular, the economists studying the question found in 2019, a 5.5-degree increase is associated with a nearly 7-per-cent decline in the asylum grant rate. “To put this into perspective,” they wrote, “in our sample, the difference in grant rate between a judge at the twenty-fifth percentile in terms of leniency, and one at the seventy-fifth percentile, is 7.9 percent.” It is difficult to imagine a given judge admitting they intended to become less lenient at higher temperatures. Yet they did.

The indirect effects of climate change on free will are just as pernicious. Natural disasters and extreme weather, for example – the hurricanes, heat waves and wildfires bearing down with ever-increasing frequency and intensity under a changing climate – act directly on neuropsychiatric health. The violence of these events is well understood to cause post-traumatic stress disorder (PTSD) in people unlucky enough to find themselves in their paths. In the wake of Hurricane Katrina, a study of low-income parents in New Orleans found that nearly half were likely experiencing PTSD, with the most extreme cases associated with the greatest degrees of exposure to the storm. Clinically, PTSD is often characterized by avoidance behaviour, detachment, a lack of enjoyment in previous interests, and irritability, among a host of other symptoms. Where is the free will in avoiding leaving your house because you are afraid of the sound of the wind?

Natural disasters also act on the unborn, with prenatal stress dumping molecules such as cortisol on the developing brain and modifying the activity patterns of neural regions responsible for decision-making, including the evolutionarily new prefrontal cortex. As Stanford neuroendocrinologist Robert M. Sapolsky details in his book Determined: A Science of Life Without Free Will, “high levels of maternal stress during pregnancy … predict cognitive impairment across a wide range of measures, poorer executive function, decreased gray matter volume in the [dorsolateral prefrontal cortex], a hyperreactive amygdala, and a hyperreactive glucocorticoid stress response when those fetuses become adults.” The developmental implications are widespread. Researchers studying a cohort of children who were in utero during Superstorm Sandy, for example, observed in these kids a dramatically increased risk of neuropsychiatric disorders such as anxiety, depression and attention deficit/hyperactivity disorder later in life – as early as preschool.

To the extent that such disorders modify one’s behaviour, whether through a willingness or ability to engage with the world or other people, they should act on our intuition of free will, as well. Young boys with conduct disorder, a condition characterized by disobedience, anti-social behaviour and aggression, are frequently disciplined at home and school as a function of their choices. The Superstorm Sandy researchers found that, relative to New York boys who were born before the hurricane or conceived after it, boys who were in utero during Sandy faced a 20-fold increased risk of conduct disorder. It would seem incumbent on us to acknowledge the influence of the hurricane when assigning any normative weight to these boys’ decisions.

“When it comes to humans,” Dr. Sapolsky writes, “it can be silly to ask what a particular gene does – only what it does in a particular environment.” Beyond the gene, it is silly to interrogate the choices of a particular person without taking their environmental context into account. In legal theory, the concept of mens rea, or the intention to commit a crime, is foundational to criminal law and presupposes the ability to make conscious, free choices. Climate change ought to spur a re-evaluation of what it means to make free choices in a world with narrowing options.

That re-evaluation might look something like this: In a universe governed by physics, climate change isn’t the difference between free will and a lack thereof, but it throws the conversation into focus. It’s still future you on trial for murder – extreme heat didn’t kill your victim. In a warming world, though, more people will die like him.

That we can acknowledge and understand the manifold relationships between environmental forces and human behaviour should make us more curious about those forces. It should prompt us to study our responses to them – academically and introspectively – as well as these decisions’ potential moral implications. It should give us reason to ask whether the legal system needs to adapt, integrating insights from neuroscience and environmental psychology to forge a new understanding of responsibility that reflects the complex interplay between human biology, environmental change and moral agency. And it ought to highlight the need for compassion: in our treatment of one another, in our evaluation of our own decisions, and in our judgment of others’.

In other words, even if free will doesn’t exist in a heat wave, moral responsibility doesn’t need to evaporate along with it. We still perceive ourselves as having free will, and that perception is enough to rescue compassion. Applied to ourselves and to our understanding of blame in others, compassion is that which affords an approach to free will that allows for mindful adjustments to our own behaviour, acceptance of what we can’t control, and the possibility of reparative and rehabilitative – as opposed to retributive – justice.

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