Climate Change, Explained

Updated: Jun 5

Both weather and climate describe the state of Earth’s atmosphere, just over different time scales: weather is what occurs day-to-day, while climate is the average over many years. “Climate change”, then, refers to long-term shifts in the average weather patterns — temperature, precipitation, humidity, wind — for a given place. Many changes in Earth’s climate have been observed so far:

More than 30 years ago, James Hansen warned Congress about the dangers of global warming; since then, overwhelming scientific evidence has confirmed that climate change is happening now, in all parts of the world, at an increasing rate.

Annual average temperature and precipitation trends based on 1895–2017

Chicago’s climate is changing, too: Average annual temperatures have increased by more than 0.8ºC (1.5ºF) since 1895, and the rate of change has accelerated in recent decades. Over this same period, total annual precipitation has increased by 15%, and the rate of heavy precipitation events has increased by 40%. Lake Michigan’s warming water has led to reduced winter ice cover and increased coastal erosion. Humidity? Increasing. Flooding? More intense. Droughts and heat waves? More frequent. It’s reasonable to wonder: What’s causing all this?

Over the past century, climate change has been primarily driven by humans burning fossil fuels — oil, gas, and coal — for energy to power our homes, transportation, and industry. This produces “greenhouse gasses” (GHG) such as carbon dioxide and methane that accumulate in the Earth’s atmosphere and trap increasing amounts of heat from the sun. At the same time, changes in land use for agriculture and housing — particularly deforestation to make space for cropland — have reduced natural ecosystems’ capacity for absorbing carbon back from the atmosphere. Although climate does vary naturally (think: El Niño / La Niña), the current rate and degree of change is unprecedented in hundreds of thousands of years, and it has been unambiguously attributed to anthropogenic (human) causes.

atmospheric carbon dioxide concentrations over the last 800,000 years

Let’s consider a specific example: Chicago, of course! According to the City’s 2017 Greenhouse Gas Inventory Report, the sum total of “community activities” within city boundaries in 2017 resulted in about 31 million metric tonnes of CO2-equivalent emissions (MMTCO2e). This is slightly down from a peak of 36.2 MMTCO2e in 2005, but it is still notably greater than zero. Of these emissions, 69% were attributed to “stationary energy”: energy consumption and direct combustion of fossil fuels (mostly gas) for residential and commercial buildings as well as industry. Transportation — planes, trains, and especially automobiles — generated 24% of emissions, while waste (mostly solid waste disposal) generated another 7%. Meanwhile, according to the 2020 Chicago Region Tree Census, the percentage of surface area covered by trees in Chicago decreased from 19% in 2010 to 16% in 2020, driven by a major loss of mature ash trees. This means that our urban canopy is sequestering less carbon, absorbing less stormwater, and blocking less daytime heat, thereby exacerbating the climate crisis in our city.

The impacts of climate change are already being felt around the world, and they’re expected to intensify as global temperatures further increase. The American West’s ongoing “megadrought” is now considered to be the worst in 1200 years; meanwhile, extreme rainfall caused unprecedented flooding in parts of Europe. The Pacific Northwest and Western Canada experienced a record-shattering heat wave; just months before, Texas experienced record cold brought south by a polar vortex destabilized by climate change. Australia’s Great Barrier Reef continues to experience more frequent and extensive “mass bleaching” events. Deforestation in the Brazilian Amazon has gotten so bad that it’s become a net emitter of carbon rather than a carbon sink. And recently it was discovered that a massive glacier in Antarctica is at risk of collapse, and it alone could raise sea levels by two feet.

Climate change is also felt here in the Midwest. Increasing temps have led to worsening air quality and heat stress; increasing precipitation has led to more frequent, damaging floods and degraded water quality; these and other environmental stressors are expected to reduce agricultural yields by mid-century. And so on.

Tundra–boreal biome shift in the presence of global warming. Source: IPCC AR5 Climate Change 2014 report

Scientists project that environmental outcomes will continue to get worse — unless we take action to prevent them. Humanity already has the knowledge and tools necessary to prevent the worst of climate change, but we lack the sociopolitical will to act with sufficient urgency. Fundamentally, we need to stop emitting greenhouse gasses into the atmosphere, and we need to stop destroying ecosystems that absorb carbon from the atmosphere, as soon and as completely as possible. This is referred to as climate change “mitigation”. There are many paths to achieving these ends, but the important thing is this: We should be doing everything, not just the “best” things, as long as they move us meaningfully closer to the goal in a sustainable, equitable manner. Collective action is key.

Governments, companies, and organizations are best equipped to implement many mitigation strategies that require top-down, systemic change:

Individuals can also play a critical role in mitigating climate change, if only by supporting and participating in the collective mitigation actions described above:

  • Electrify your current home, and make it more efficient, with a retrofit; if building a new home, make it net-zero from the get-go. It’s okay to do this piece-by-piece: For example, next time you buy an appliance, choose an efficient, electric option like a heat pump or induction range over fossil-fueled alternatives.

  • Prefer to walk, bike, scoot, or take public transit when traveling around the city; if you need your own vehicle, buy electric — and consider carpooling. When traveling longer distances, take a train instead of an airplane; or, if possible, skip the trip and just video conference.

  • Reduce your personal waste by eating more plants and less meat, and composting what you don’t eat. Consider growing your own food and/or herbs. Minimize the amount of plastic you use (plastics are derived from fossil fuels), and recycle as much of the rest as possible.

Actively support those policymakers, business leaders, and organizations pushing for more sustainable, carbon-neutral systems through your vote, your pocketbook, your endorsement, and your time. Consider personally divesting from fossil fuels. Don’t underestimate the impact of democratic and market pressures on bad actors!

Future emissions scenarios, including Paris climate pledges. Source: Hannah Ritchie and Max Roser (2017) - "CO₂ and Greenhouse Gas Emissions". Published online at

Because climate change is already causing significant harm, “adaptation” is also required to respond to climate impacts and reduce the risks to social, economic, health, and environmental assets and systems. Adaptation strategies vary according to a particular domain’s hazards and vulnerabilities, but it’s typical to mix options from a few general categories:

  • Engineered / technological: seawalls and levees, improved storm and wastewater management, efficient irrigation and other water-saving technologies, early warning systems, passive cooling of buildings, …

  • Ecosystem-based: conservation and reforestation, assisted migration of species, fisheries co-management, green infrastructure like parks and a healthy urban canopy, …

  • Educational / informational: awareness campaigns, improved curricula and expanded access to education, hazard and vulnerability mapping, improved forecasts and systematic monitoring, participatory research, …

  • Policy / regulatory: disaster planning and preparedness, municipal water management programs, land rezoning, improving building standards, establishing natural protected areas, fishing/hunting quotas, insurance requirements, …

Increasing evidence suggests that adaptive action by local governments and the private sector will be particularly important moving forward. And though the preference so far has been for “incremental” approaches, it’s clear that more transformational adaptation will be required soon to keep ahead of our changing climate.

Landmark international treaties and gatherings like the Paris Agreement and COP26 have secured some progress over the years, but thus far have not been enough to “bend the curve” on global GHG emissions. Meanwhile, the U.S. Congress hasn’t passed substantive climate legislation in decades, and its 2021 attempt died in the Senate. Instead, the U.S. has relied largely on a patchwork of Executive actions and regulation enforcement (when not actively dismantling them) to push forward climate-friendly policy. This hasn’t been very effective.

Trends in Atmospheric CO2 vs Global Temperature Change

In contrast, Illinois has made significant recent progress through the Climate and Equitable Jobs Act (CEJA), and is now working to fund and implement those reforms. The City of Chicago is currently pursuing — or planning to pursue — a variety of climate change mitigation and adaptation strategies. In 2008 Chicago unveiled a Climate Action Plan followed by a Sustainable Action Agenda in 2012, and it’s currently developing an updated CAP for 2022. A notable infrastructure project to reduce flooding and sewage overflows, the Tunnel and Reservoir Plan, is mostly operational as of 2017, but its cost and shortcomings make it more of a cautionary example for municipal adaptation efforts. Collectively, these initiatives are accelerating the shift to renewable energy and electric vehicles, expanding carbon-absorbing green spaces, and promoting environmental justice and resiliency, but there’s still much more to be done. To see who’s leading the charge, check out local and state politicians’ environmental records (scored by the Illinois Environmental Council, an EEC affiliate) here and here.

Climate change is a serious, growing problem, and it’s easy to become overwhelmed by the scale of the challenge, which can lead to inaction and doomerism. Don’t fall into that trap! Every bit helps — every reduction in emissions, every tenth of a degree shaved off the global trajectory, every increase in capacity to adapt. In other words, “preventing” climate change isn’t a binary outcome, it’s a spectrum of possibility. There are reasons to be hopeful, from the accelerating rate of renewable energy installation and electric vehicle adoption to increasingly ambitious national and international climate plans. But more and more urgent action is needed, now. Keep doing your best, and continue supporting others so they can do their best, too.

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