How CO2 and Climate Shape Plants: C3, C4, and Greening

Introduction

Plants play a big role in Earth’s climate, and their growth is influenced by CO2 levels and weather patterns. Two main types of plants, called C3 and C4, respond differently to these changes, affecting what grows where—from ancient grasslands to modern farms. This page explores how CO2 and climate have shaped these plants over millions of years, using clues from fossils. We’ll also look at how rising CO2 today (around 420 parts per million, or ppm) is causing more plant growth, known as “global greening,” and how this extra growth can fuel wildfires in places like Chile and California when wet seasons turn dry.

What are C3 and C4 Plants?

Plants use sunlight to turn CO2 and water into food through a process called photosynthesis. C3 and C4 plants do this differently, which affects where they grow best. C3 plants, like wheat, rice, beans, and all trees, use a simpler process but need more water and do better in cooler, wetter areas. C4 plants, like corn, sugarcane, and many grasses, are more efficient with water and thrive in hot, dry places. A third type, CAM plants, like pineapples, save water by opening their pores at night, making them ideal for deserts. C3 plants benefit more from higher CO2 levels, growing faster and using less water, while C4 plants are less affected but dominate in warmer climates.

Past Climate and Plant Shifts

Millions of years ago, Earth’s climate shaped what plants grew where. From 23 to 5 million years ago, during a time called the Miocene, the planet cooled and dried, even as CO2 levels rose from about 280 to 350 ppm. Fossils of tiny ocean creatures and plant silica show that forests shrank, and grasslands spread. Around 6 to 7 million years ago, C4 grasses began to take over from C3 grasses in many areas. This wasn’t because of CO2 levels but because cooler, drier, and more seasonal climates favored C4 plants, which handle heat and drought better. The higher silica in C4 grasses also caused some mammals to go extinct, as their teeth wore down faster. So, while CO2 rose, it was climate—temperature and dryness—that mainly drove the shift to C4 plants.

Modern Greening and Its Effects

Today, CO2 levels are around 420 ppm, up from 260–340 ppm before the Industrial Revolution. This rise has caused global greening—more plant growth worldwide. By 2023, satellite data show a 7% increase in leaf area since 2000, largely because CO2 helps plants grow faster. C3 plants, like wheat and soybeans, benefit most, with 10–20% higher yields, while C4 plants, like corn, gain less, around 5%. In the Arctic, tundra vegetation has increased 30% since 1980, supporting more wildlife. Even dry areas in Australia are greener, as plants use water more efficiently with higher CO2.

But this greening has a downside. More plant growth can mean more fuel for wildfires, especially in places with wet and dry seasons. In California, heavy rains in 2022 and 2023 grew lots of grasses and shrubs, but a dry 2024—only 0.16 inches of rain in Los Angeles since October—turned them into tinder. This fueled the 2025 Palisades and Eaton fires, which destroyed over 16,000 structures. In Chile, rains in 2022 from a weather pattern called El Niño led to more plant growth, but heatwaves and a dry 2023 sparked fires that burned 1.7 million hectares over the past decade. These wet-to-dry swings, which have increased 31–66% globally since the 1900s, show how greening can worsen fire risks when seasons shift.

Despite fears of mass starvation due to climate change, global crop yields have hit records in recent years, like in 2017–2018, thanks to CO2 boosting C3 crops. While greening has challenges, like wildfire risks, it also brings benefits, like more food and wildlife habitats, showing CO2’s complex role in nature.

Stomata are tiny pores on plant leaves and stems that regulate gas exchange, allowing carbon dioxide in for photosynthesis and releasing oxygen and water vapor. Their number varies due to CO2 concentrations. "Data from various stomata studies (ref. 10-20) show CO2 concentrations over the last 11,000 years varied between 260 and 340 ppm (average: 305 ppm). In contrast, the Dome C ice core record shows no significant variability and considerably lower overall CO2 levels (average: 270 ppm)." See The CO2 Record in Plant Fossils

Stomata as Climate Records

Plants have tiny pores called stomata that let in CO2 and release water through a process called transpiration. When CO2 levels are higher, plants need fewer stomata and lose less water, which helps them grow more efficiently. Fossils of plant stomata act as a clue to past CO2 levels. Studies show that before the Industrial Revolution, CO2 varied between 260 and 340 ppm, averaging 305 ppm, not a steady 280 ppm as some ice core data suggest. Today’s higher CO2—about 420 ppm, a 25% increase from human activity—has boosted plant growth, especially for C3 plants, but the benefits depend on climate conditions like temperature and rainfall.

Conclusion

CO2 and climate have shaped plants for millions of years, from the rise of C4 grasses in the Miocene to today’s global greening. While higher CO2 levels—now at 420 ppm—help C3 plants like wheat grow more with less water, they also increase plant growth that can fuel wildfires in places like Chile and California when wet seasons turn dry. History shows climate, not just CO2, drives plant changes, and today’s greening brings both benefits, like higher crop yields, and challenges, like fire risks. Understanding these patterns with hard data, not unproven predictions, helps us see the full picture of CO2’s role in nature.

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