Understanding our climate
- Devika Bakshi
Inherited stability
The first humans were born into a difficult environment. Over tens of thousands of years the earth’s climate kept changing, with gradual shifts from very cold to mild, and back again. Wherever our ancestors went, they lived in relationship with the forces of nature, aware and engaged in the physical world.
There are still communities of people on Earth today who observe ancient traditions and maintain a deep connection with their surroundings, but most people turned away from this kind of intimacy with the natural world. What prompted our ancestors to stop moving, settle, and develop infrastructure and agriculture? A miraculous bit of good fortune. Around 12,000 years ago, the climate stabilized, and almost no place on Earth was too hot or too cold for humans to live and thrive.
Since then, there have been variations in temperature and precipitation from year to year, but the averages stayed the same, as did the extremes. Keeping records of temperatures, rainfall, storms, tides, and seasons allowed us to anticipate what was most likely and know what rare events to guard against. We came to count on these regular patterns and built civilizations around them thousands of years before anyone understood why the climate was stable.
Civilization
In using “civilization”, Probable Futures refers to everything familiar to and associated with modern human life, from complex societal structure, cultural practices and daily comforts to technology and economies. The maintenance of all these structures requires a stable climate.
Civilization is often defined as the development of human life into complex societies and cultures similar to what exists today. Depending on definition and interpretation, various thresholds are considered to indicate the start of civilization, including the development of agriculture, social stratification, language, metropolitan societies and the keeping of written records.
Archeology indicates that widespread agriculture began around 12 thousand years ago, thanks in part to changes in climate that favored annual plants like grains and root vegetables. Simultaneously, records show humans constructing the first megaliths and permanent settlements. The development of agriculture based on a hospitable, stable, and thus predictable, climate, combined with division and specialization of labor, provided humans with leisure time to engage in activities like art and music. Underpinning all these advancements is a reliable climate.
Assumptions of a stable climate are now built into everything around us. Farmers and city dwellers in dry regions rely on distant mountain snowmelt for irrigation and drinking water. There are communities that migrate during the year to follow predictable patterns of weather and animal migrations for sustenance and survival. Power systems and roads are engineered to withstand a limited number of days above specific temperature thresholds. Every community has cultural touchstones, from specific foods at different times of the year to ceremonies, sporting events, and celebrations that rely on historical patterns of weather repeating.
Explore the history of our planet’s temperature 
People began keeping consistent temperature records around 1850 as thermometers became common. Large scale fossil fuel use didn’t begin until the early 20th century, so 1850-1900 is a common baseline for pre-industrial temperatures. In the 1970s, scientists began warning that the climate was starting to warm and that temperatures would rise if people continued to put carbon dioxide and methane into the atmosphere. 
By studying air trapped in glacial ice, scientists have been able to reconstruct the temperature history of earth’s atmosphere. Those records show that average temperatures observed in 1850-1900 were a continuation of climate stability that began around 10,000 BCE. This epoch, known as the Holocene, is an “interglacial” period, or a warm part of an ice age, as it is just cold enough to support glaciers. 
In this mild environment, nowhere on Earth was too hot for humans to live, and the stability of the climate made weather patterns predictable. This predictability made life easier for our ancestors who seized this good fortune by settling, developing what we know as civilization, and multiplying. 
The first people like us were born into a harsher, more unstable period of the current ice age. This ice age period, known as the Pleistocene epoch, was characterized by swings from extremely cold glacial maximum to brief, mild interglacials. These swings were caused by slow changes in the earth’s tilt and orbit. 
If we hadn’t burned so many fossil fuels and cut down so many forests, the atmosphere would now be cooling, resuming the patterns of the Pleistocene. Instead, our emissions have caused temperatures to rise to the edge of the civilized range. Glaciers and sea ice are rapidly shrinking. 
Until recently humans were the only cause of warming. Higher temperatures, however, have begun enabling huge forest fires and thawing permafrost, both of which push more greenhouse gases into the atmosphere, causing yet more warming. Simultaneously, the retreat of glaciers and sea ice exposes more dark land and sea to sunlight, further raising temperatures. 
Evidence in fossils, sediment, and other sources have enabled scientists to reconstruct temperature records from before our ice age. They offer a strong warning: the atmosphere can get much warmer. Outside the bands of our civilized era lie the risks both of continual warming and of climates that reptiles, amphibians, and other heat-loving creatures would enjoy, but that would be toxic for warm-blooded creatures like us.
Source: 100M-15.9 BCE: Friedrich et al (2012) & Hansen et al (2013); 15.9M BCE-722k BCE Zachos et al (2008) & Hansen et al (2013); 722 k BCE-2011 EPICA A, HadCRUT 5, and CMIP5 RCP8.5
What governs Earth’s climate
Sunlight carries energy toward Earth. Clouds reflect about ¼ of this energy back into space, some is absorbed by the atmosphere itself, and the remaining half hits the surface of the earth. Snowy glaciers and Arctic Sea ice are highly reflective, while the dark blue ocean absorbs heat the way dark clothing does.
Credit: Berke Yazicioglu
The absorbed energy heats the earth. Some of that heat radiates back toward space. Gases including carbon dioxide (CO2) and methane (CH4) trap some of that heat. If they didn’t, nights would be incredibly cold.
Where we are today
The glorious stability we rely on is being endangered by human activity, particularly the burning of fossil fuels. The amount of solar energy reaching the earth has been steady in recent decades, while atmospheric CO2 has increased dramatically, trapping more heat.
Perhaps the most straightforward example of the effects of more greenhouse gases is that nights, when the heat of the day dissipates, no longer cool off as much. For example, over the last 30 years in the United States, there have been twice as many instances of record high nighttime temperatures as record high daytime temperatures.
Warmer temperatures also disrupt seasonal patterns, with winter starting later and spring earlier. A poignant example of this lies in a seasonal and cultural event known throughout the world. For over 1,000 years, the people of Kyoto, Japan have recorded the date at which the local cherry trees reach their peak blossom, which consistently occurred around April 15. In 2021, the peak of cherry blossoms in Kyoto was reached on March 26, earlier than any year in recorded history.
Warming doesn’t just change temperatures, but all weather phenomena. Rising temperatures affect rainfall: When air warms by 1°C it can hold 7% more moisture. Warmer air pulls more moisture from the land (and from plants, animals, and our bodies, in the form of sweat) and tends to release it more rapidly. As a result, both droughts and deluges are becoming more common.
Approaching tipping points
Over hundreds of millions of years, sunlight fueled plant growth, some of which was buried in swamps and seabeds and eventually compressed into carbon-rich coal, oil, and methane. Carbon is also locked in trees, plants, and soil and trapped in frozen land called permafrost in the Arctic.
Permafrost
The Arctic is home to enormous expanses of frozen soil called permafrost. Permafrost soil is a combination of ice mixed with the remains of plants that grew briefly one summer and then were frozen and buried before they fully decomposed, depositing their carbon. This vault of millions of years of stored carbon has been growing over time. Until recently, temperatures in the Arctic rarely warmed enough for this permafrost to thaw, ensuring that the stored carbon remained locked up. New annual deposits of carbon from decaying plants were added—with little being released—building a wealth of permafrost carbon stored across the vastness of the Arctic. Now rising temperatures are prompting the steady thawing of permafrost, releasing this carbon. The slow rate of permafrost growth, combined with dwindling freezing temperatures, means permafrost cannot form and lock in carbon at the rate it is being released.
When these stores are unlocked by heat, they release this ancient carbon, causing further warming. That accelerated warming can create large forest fires or thaw frozen tundra, which releases yet more carbon and further increases warming—this is known as a biotic feedback loop.
Biotic feedback
Biotic feedback describes a looped response between plant-based ecosystems and the atmosphere that helps determine the stability of Earth’s climate. In a stable climate, growing plants absorb atmospheric CO2 and release this carbon eventually because of decay (or another natural process) in proportion with plant regrowth rates. Warming temperatures due to human emissions make plants release carbon more quickly by accelerating decay (examples include thawing permafrost and larger and more frequent forest fires). The resulting carbon release is more rapid than the capture of carbon that occurs when plants regrow. Essentially, humans have made the world so hot by filling the atmosphere with heat-trapping greenhouse gases that plants now release carbon too quickly for growing plants to absorb the carbon and keep up. These biotic feedbacks contribute CO2 to the atmosphere, generating further warming.
Scientists who foresaw the consequences of increased greenhouse gases understood from the paleoclimate records that such feedback loops were a risk, but they seemed far away. Even 10 years ago, most scientists believed that such loops would not start below 2°C. That is one of the reasons why many current climate models do not yet include this dynamic.
Climate models and biotic feedbacks
Climate models, including the ones used by Probable Futures, do not incorporate biotic feedbacks, such as the thawing of permafrost into their projections for future warming scenarios. This means all models underestimate warming by only considering human emissions. The additional emissions from thawing permafrost or carbon release from plants may have crucial implications for Earth’s delicate climate system. For example, in a Hothouse Earth scenario these natural emissions would create cascading natural impacts generating even further warming.
In the last few years, however, permafrost has begun to thaw at alarming rates and forests have begun to burn in larger, hotter fires during more months of the year, including in places that rarely used to burn. The vast trapped stores of carbon are starting to be released into the atmosphere faster than natural processes can trap them. We thus stand to irreversibly lose stability. We are unprepared for this prospect.
Most considerations of the future still contain the assumption that even if a warmer atmosphere will pose challenges, temperatures will again stabilize as soon as we stop emitting carbon, perhaps at 2.5°C or 3°C or even 4°C above the stable temperatures we inherited. This assumption is understandable, as our recorded history made stability seem fundamental, but biotic systems are already adding to human emissions and creating more momentum for warming. Facing this unprecedented challenge requires a mindset that prioritizes risk. The good news is that we know the sources of rising risks and have the power to affect them.
Risk mindset
Risk is a combination of the probability of an event happening and the gravity of its negative consequences.There are many forms of physical, societal, and economic risk associated with climate change. Probable Futures focuses on encouraging a risk mindset in the following categories.
Existential risk considers the likelihood of perpetual instability, in particular continued warming towards a Hothouse Earth pathway resulting in conditions that cannot sustain human life.
Physical risk refers to the potential for negative impacts caused by climatological conditions. These can be acute, such as a singular flooding or drought event. They can also be chronic, such as consistently high temperatures or changes to entire ecosystems.
Higher order risk refers to societal responses to climatological impacts, such as human migration or supply chain disruption.
Model risk considers the possibility that models and methodologies used to assess risk could have inherent flaws, leading to flawed decision making. For example, if you are not considering climate risk in your decision making currently, you are using a “model” that assumes with certainty that climate change will have no impact.
The probability of limiting global warming to 1.5°C is approaching zero. This graphic, based on information from the IPCC and the Global Carbon Project, illustrates the pace and scale that emissions would need to drop in order to make 1.5° or 2°C more likely than not.
Imagine and prepare
It can be hard to picture carbon atoms, solar energy, and average global temperatures, but we all have the experience necessary to envision climate outcomes. Imagine hot, humid nights, melting ice, deluges instead of gentle rain showers, drought, and oddly warm winter days.
Fortunately, we understand our Earth and its systems well enough to be able to make projections of what will happen in a warming world. Over the past 40 years, climate models have accurately predicted the scope and scale of how our global climate would change. Even the best science available isn’t perfect, and the further our climate moves from its historical norm, the more difficult it becomes for climate models to accurately project extreme conditions. At their best, models help us envision what is coming well enough to plan for the futures we likely cannot avoid, and motivate action to avoid the futures that could include unmanageable outcomes.
Climate models do not inherently predict when specific temperatures will be reached. Instead, they tell us at what concentrations of atmospheric CO2 and methane (CH4) different temperatures and weather conditions will occur. Therefore, instead of using the models to predict timing, we use them to ask: “What weather phenomena do the models predict at different levels of atmospheric warming like 1.5°C, 2.0°C, and 3.0°C?” This approach lets the models work as they are meant to, and highlights the fact that when—and if—different levels of warming are reached depends on what we humans do.
When examining a map of climate projections, like the ones on this platform, consider connections across space. We have all seen weather move from one area to another. We have all felt how political, economic, and public health events that originate in one location can affect us far away. If you prepare your house for changes in the local climate, but your neighbors, city, or electric company don’t, are you actually prepared?
There are also many connections we can’t easily see but that make the natural systems around us work, including animal migration patterns, the movement of nutrients through the air and water, and ocean currents. For these reasons it is most useful to investigate climate change using global maps that allow you to zoom in to continents, regions, and local areas. If you only look at global maps you miss important changes, but if you only consider local conditions you are likely to miss important connections.
It begins with heat
We stand to lose something precious if greenhouse gas concentrations continue to grow. Heat will determine the future of our planet and our civilizations.
