7.13: Climate Change

One of the most pressing concerns in environmental health is the effects of climate change on the planet, and human and animal populations. Climate change refers to the shifts in climate over time in various regions across the globe. Many of these shifts are due to global warming, or the observable increase in average temperature of the earth. The term “global warming” sometimes causes confusion because people think it means all temperatures will be higher throughout the year. Instead, it’s better to think of global warming as the driver of climate change, because the increasing average global temperature is causing shifts to different regional climates across the globe - including more severe storms, droughts, and permanent changes to ecosystems.

Figure \(\PageIndex{1}\): Change in Average Temperature With Fahrenheit. (Copyright; CC0, NASA’s Scientific Visualization Studio, Key and Title by uploader

How do we know that the earth is warming? Evidence from ice cores collected from Greenland and Antarctica, plus ambient temperature data collected since 1958 demonstrate that although the earth has gone through cycles of warming and ice ages, human activity since the industrial revolution has caused much more warming in the last 150 years than occurred in the ~800,000 years prior. Images from space show ice sheets and glaciers melting and shrinking, dumping fresh water into the ocean. And since the middle of the last century, scientists have tracked a 2 °F increase in the average temperature of the planet, with the most significant increase happening in the last 4 decades (NASA, 2024).

Figure \(\PageIndex{2}\): Melting Canadian glacier. (Katherine Lean, CDC/ CDC Connects, 2017.)

This glacier located along the Icefields Parkway, near Banff National Park, Canada, is melting, showcasing the effect global climate change is having on the environment. The glaciers in this area have been studied since the 1950s, making them some of the longest scientifically observed glaciers in the world. Photo Credit:

This most recent and rapid global warming is considered to be anthropogenic (caused by human activity), not something that would typically happen from natural sources alone. Anthropogenic factors include the release of Greenhouse Gasses (GHG). These GHGs include carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O),and are released into the atmosphere from the burning of fossil fuels for energy, as well as from modern agricultural practices. Trees actually remove CO₂ from the atmosphere and store it in their trunks - so forests become “carbon sinks”. Deforestation therefore also contributes to global warming by cutting into carbon sinks - thus decreasing the amount of CO₂ collected (Burke & Weill, 2023). These GHGs left floating in the atmosphere trap heat from the sun and reflect it back down to the earth, causing an increase in the average temperature - much like a greenhouse uses glass walls to trap heat from the sun and keep the interior warmer for plants. Since the 1970s, the release of GHGs has increased much faster than in years prior, with the largest spike happening after the turn of the 21st century (Frumkin, 2016). Part of this is due to global population growth, which certainly contributes to the demand for energy production and thus the burning of fossil fuels to make it. However, the largest contributors to GHG tend to be the wealthiest countries, and per-capita GHG emissions are higher for more economically developed countries than for less economically developed countries. So it’s not just that more people require more energy, it’s also the behaviors and demands of wealthy societies that are driving GHG emissions. Still, as a few countries like Germany and the Netherlands have demonstrated, quality of life can be maintained while reducing GHG emissions through the use of technology and renewable energy sources (Ritchie & Roser, 2020). See figures \(\PageIndex{3}\) and \(\PageIndex{4}\) below.

Figure \(\PageIndex{3}\): Annual CO₂ emissions from fossil fuels and industry. (Copyright; CC-BY Our World in Data)

Figure \(\PageIndex{4}\): CO₂ emissions per capita vs. gross domestic product (GDP) per capita. (Copyright; CC-BY Our World in Data)

Increasing the Earth’s average surface temperature has several consequences. When the ocean warms even just a little, it expands. As ice sheets and glaciers on land melt, they also contribute to rising sea levels. (Melting icebergs don’t contribute to increased sea levels, as their volume is already accounted for - like ice cubes in a glass of water). Increases in ocean temperature can cause the “bleaching” of coral reefs, essentially killing them and the marine life that rely on them for food and shelter (NOAA, 2023). On land, the higher temperatures cause more evaporation of water from plants and soil, leading to droughts. Warming air can also hold onto more water vapor, which can also cause more extreme weather - like severe rainstorms, hurricanes, and flooding (Frumkin, 2016). As warm air travels toward the poles, it shifts the cold air from the polar vortex (low pressure air at the north and south poles) which can cause severe winter storms - like the unexpected snowstorms experienced in Texas in 2021 which caused massive failures in the power grid (Burke & Weill, 2023). While some rain is needed for vegetation growth, very wet springs followed by dry summers cause all that vegetation to dry out. In 2021 during the summer, dry timber and windy conditions led to mega wildfires in the Sierra Nevada mountains (Sierra Nevada Conservancy, 2022). Droughts can not only dry out plants, but also make the soil less permeable, causing flooding when substantial rains do fall, and allowing precious rainwater to wash away into the ocean instead of sinking into groundwater aquifers (Fry & Reyes-Velarde, 2019). And if heavy rains follow a wildfire, they can cause mudslides that change terrain and threaten homes and roads (Burke & Weill, 2023). .

Climate change might include changes in temperature, rain or snowfall, drought, or wind that influence the overall climate for that region. Climate is a term used to describe average weather patterns. Southern California for example has a mostly mediterranean (arid or semi-arid) climate around the coastal cities and Los Angeles, gradually shifting to a desert climate in the Mojave desert and inland regions. Los Angeles has pretty mild temperature changes over the seasons, with some rainfall in the winter, and dry, sunny summers. The desert areas get much less rain of course, and have more temperature extremes, getting hotter in the summer and colder in the winter. Weather is made up of the temperature, atmospheric pressure, precipitation, wind, and humidity at any given time and can change rapidly. Certain weather patterns may be predictable for a particular climate, but the two are not the same by definition. Over time, if the climate changes in a specific area, there may be more extreme weather patterns, and there will be changes in the overall ecology of that region - which may positively or negatively influence its ability to support life of the plants and animals who live there (Burke & Weill, 2023).

In our California example, the average temperature in the Los Angeles (LA) region has already increased since the middle of the 20th century, and is expected to be 4-5 °F warmer by the middle of the century. This increase in temperature is causing more of the snowpack to melt from the nearby mountains, and more of the water sources across the state to evaporate. The combination of warm, dry summers and substantial rainstorms in the winter will cause more vegetation to grow in the spring and become dry, wildfire fuel in the summer. Subsequent rains will increase the risk of mudslides and soil erosion. Along the coast, higher sea levels will threaten coastal communities, which, along with mudslides can cause severe danger to lives and homes (California Climate Adaptation Strategy, 2024). Central California has a mediterranean climate similar to LA, which is conducive to growing fruit and nut orchards and crops that require rain in the winter and warm sun in the summer. If the entire climate in central CA becomes more desert-like, it will severely impact the state’s large agricultural economy. For example, almond orchards in CA produce 80% of the world’s almonds - but these trees require many years to mature and need water year round (Chea, 2021).

The distinction between climate and weather is an important one. Climate is the average of weather conditions (temperature, rainfall, etc.) over a 30-year period. Weather is what we call the atmospheric conditions at any given moment -- sun, rain, heat, snow, etc. The focus on climate over weather in studying what's happening on earth is akin to public health's focus on population health over individual health. For example, a 5% increase in the number of days an individual misses work or school due to illness might not be a big deal -- but a 5% increase across a population is a very big deal. Similarly, while any given day in San Francisco the temperature goes up or down 2 degrees Celcius (about 5 degrees Fahrenheit) and no one thinks twice, when the climate heats up by 2 degrees Celsius, it is a very big deal indeed.