Climate Change, Ocean Acidification, and Ecosystems

Climate change is a long-term shift in global or regional climate patterns, and the present warming is caused mainly by the increase of greenhouse gases such as carbon dioxide from burning fossil fuels; ocean acidification is the fall in the pH of seawater as the ocean absorbs part of this carbon dioxide. In brief: the global average surface temperature has risen by roughly 1.1 to 1.2 °C since the late 1800s, atmospheric CO₂ has increased from about 280 ppm to over 420 ppm, and the surface ocean pH has fallen by about 0.1 units, which means about a 26 percent increase in hydrogen ion concentration. Ecosystems respond with shifts in range, timing and community structure.

Greenhouse gases keep the Earth warm enough for life, but extra emissions strengthen this effect. This guide explains the greenhouse effect and the evidence for warming, the chemistry of ocean acidification, the effects on land and marine ecosystems, compares the effects in a table, works through pH and CO₂ calculations and discusses solutions and common mistakes.

What is the greenhouse effect?

The Earth receives energy from the Sun, mainly as visible light, and warms. The warm surface emits infrared radiation. Greenhouse gases in the atmosphere absorb part of this infrared radiation and re-emit it in all directions, including back toward the surface, which keeps the average temperature at about 15 °C instead of roughly −18 °C. The main gases are water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and fluorinated gases. The natural greenhouse effect is essential for life. The enhanced greenhouse effect comes from human emissions:

• CO₂: from burning coal, oil and gas, and from deforestation and cement production. It is the largest contributor, and remains in the atmosphere for centuries to millennia.
• CH₄: from livestock, rice fields, landfills, and leaks from gas production. It is more powerful per molecule than CO₂, but stays in the atmosphere for about a decade.
• N₂O: from fertilizers and some industrial processes.

What is the evidence for climate change?

• Temperature: independent records from land stations, ships, satellites and buoys show warming, and the last decade was the warmest in the instrumental record.
• Carbon dioxide: measured at Mauna Loa since 1958 (the Keeling curve), and in air bubbles in ice cores for the previous 800,000 years. Concentrations are now well above the natural range in the cores of about 180 to 300 ppm.
• Isotopes: the changing ratio of carbon isotopes shows that the added carbon comes from fossil fuels.
• Ice and sea level: glaciers and the Greenland and Antarctic ice sheets are losing mass, Arctic sea ice has declined, and sea level has risen by more than 20 cm since 1900 because of the melting of ice and the thermal expansion of water.
• Ocean heat content has increased, and the oceans have absorbed most of the extra heat.

The Intergovernmental Panel on Climate Change (IPCC) states that it is unequivocal that human influence has warmed the atmosphere, ocean and land.

What is ocean acidification?

The ocean absorbs roughly a quarter of the CO₂ emitted by human activity. In seawater, the CO₂ reacts with water:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

The increase in hydrogen ions (H⁺) lowers the pH. The hydrogen ions also react with carbonate ions (CO₃²⁻) to form bicarbonate, which reduces the carbonate available to organisms that build shells and skeletons from calcium carbonate (CaCO₃). The average pH of surface seawater has fallen from about 8.2 before the industrial era to about 8.1 today. The water remains slightly basic (pH above 7), so “acidification” refers to the direction of change.

• Calcifiers affected: corals, oysters, mussels, sea urchins, pteropods (small free-swimming snails) and some plankton find it harder to build shells and skeletons, and in more acidic water shells may dissolve.
• Other effects: experiments suggest that some fish have altered behavior and sensory function at elevated CO₂, though results vary among studies and species.
• Coral reefs are also threatened by warming, which causes bleaching: corals expel their symbiotic algae (zooxanthellae) under heat stress and may die. Major global bleaching events have occurred in 1998, 2010, 2014 to 2017 and 2023 to 2024.

How does climate change affect ecosystems?

• Range shifts: species move toward the poles and up mountains. Many birds and butterflies in Europe and North America have shifted their ranges tens of kilometers, and mountaintop species have nowhere higher to go.
• Phenology: spring events occur earlier. If predators and prey, or plants and pollinators, respond differently, a mismatch can occur. In the Netherlands, pied flycatchers arrive from Africa when caterpillar peaks are earlier in some woodlands, with declines in the birds.
• Polar ecosystems: loss of sea ice reduces the hunting platform of polar bears and the algae living under the ice.
• Extreme events: heat waves, drought, wildfire, floods and storms are changing in frequency and intensity.
• Oceans: warming reduces oxygen and increases stratification, and fish stocks move to cooler waters.
• Extinction risk: the IPCC has concluded that the risk increases with the level of warming. Species with narrow tolerances, limited dispersal or specialized habitats, such as amphibians in cloud forests and corals, are particularly at risk.
• Feedbacks: melting permafrost can release methane and CO₂, and loss of ice reduces reflectivity (albedo), and both amplify the warming.

Effects on ecosystems compared

How can we respond?

• Mitigation: reducing emissions with renewable energy, efficiency, electrification of transport, less methane, protection of forests, and carbon capture. The Paris Agreement (2015) aims to hold warming well below 2 °C, and to pursue 1.5 °C.
• Adaptation: protecting corridors and refuges, restoring wetlands and mangroves, and managing water and fire.
• Nature-based solutions: restoring forests, peatlands and seagrass, which store carbon and support biodiversity.

Worked example: pH and CO₂ calculations

1. pH definition: pH = −log₁₀[H⁺], so [H⁺] = 10^−pH.
2. Change of 0.1 pH units: from pH 8.2 to 8.1, the ratio of hydrogen ion concentrations is 10^{8.2 − 8.1} = 10^0.1 = 1.259. This is an increase of about 26 percent.
3. A further fall: from pH 8.2 to 8.05 (0.15 units), the ratio is 10^0.15 = 1.413, an increase of about 41 percent. A change of 0.3 units would double the concentration of H⁺, since 10^0.3 ≈ 2.
4. CO₂ rise: from 280 ppm to 420 ppm, the change is (420 − 280) ÷ 280 = 0.5, so a 50 percent increase.

Because the pH scale is logarithmic, a small change in pH represents a large percentage change in H⁺ concentration. The ocean is not acidic in the strict sense: water at pH 8.1 still has a lower concentration of H⁺ than pure water at pH 7 (about 10^−8.1 compared with 10⁻⁷ mol/L).

How was climate change discovered?

Joseph Fourier described the atmospheric warming effect in the 1820s. John Tyndall measured the absorption of infrared radiation by gases in the 1860s, and Svante Arrhenius calculated in 1896 that doubling CO₂ would warm the planet by several degrees. Charles Keeling began continuous CO₂ measurement at Mauna Loa in 1958. James Hansen testified to the US Congress in 1988, and the IPCC was founded in the same year. Ocean acidification was named in 2003 by Ken Caldeira and Michael Wickett, and the Paris Agreement was adopted in 2015.

Which mistakes are common?

Mistake 1: Confusing weather and climate

Weather is the state of the atmosphere over a short time. Climate is the average over decades. One cold day does not disprove warming.

Mistake 2: Saying that the ocean is becoming acidic

Seawater is still basic. It is becoming less basic, which is why the term acidification is used.

Mistake 3: Confusing the ozone hole with the greenhouse effect

The ozone layer is a separate issue related to ultraviolet radiation.

Mistake 4: Assuming that natural cycles explain the recent warming

Solar and volcanic changes have been measured, and they cannot account for the observed warming. The pattern of warming (warming of the lower atmosphere and cooling of the stratosphere) is consistent with greenhouse gases.