The Science Behind a Cleaner Future

Level 1 · Understand the basics

Energy is the capacity to cause change or do work. It exists in many forms, including light, the chemical energy stored in food, motion and heat. It can be transferred from one place to another, and transformed from one form into another.[12]

Sunlight supplies the energy for most ecosystems. Plants, algae and other photosynthetic organisms capture light energy and store it as chemical energy in sugars and other organic molecules. Animals obtain energy by eating plants or other animals, so energy moves step by step through food chains and food webs.[12]

At every transfer, organisms use much of the energy they take in to grow, move and stay alive, and that work releases heat into the environment. The energy is not destroyed: energy is conserved. Once it is dispersed as heat, however, living things can no longer use it. That is why ecosystems need a continuous supply of energy from the Sun, while matter such as carbon and nitrogen is recycled again and again.[12]

Level 1 · Understand the basics

Energy from the Sun reaches Earth mostly as visible light. Some is reflected back to space by clouds, ice and other bright surfaces. The rest is absorbed, warming land, ocean and air.[2][13]

A warm surface gives off energy as thermal infrared radiation. Greenhouse gases, including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and water vapour, absorb some of this infrared radiation and emit it again in all directions, including back toward the surface. This slows the loss of energy to space; it does not stop it.[2][13]

This natural greenhouse effect is essential for life as we know it. NASA describes it as keeping Earth’s average surface temperature near 15 °C, and gives a simplified estimate: if carbon dioxide were removed, the natural greenhouse effect would collapse and the surface would be roughly 33 °C colder. That figure describes losing the whole natural effect, not the direct warming of carbon dioxide alone. Water vapour, which NASA describes as a feedback that amplifies warming, also contributes.[13]

The enhanced greenhouse effect is different. Human activities, primarily burning fossil fuels, have increased concentrations of long-lived greenhouse gases. More outgoing infrared is absorbed and re-emitted, so Earth temporarily releases less energy to space than it absorbs, and the climate system warms.[13][14][16]

Water vapour is the most abundant greenhouse gas, but in today’s warming it acts mainly as a feedback: a warmer atmosphere holds more moisture, which amplifies warming started by other causes. Carbon dioxide and other long-lived gases act as primary drivers.[13]

Level 1 · Understand the basics

Carbon is the chemical backbone of life. It is found in every living cell, in the carbon dioxide we exhale, in the ocean, in soils and in rocks such as limestone. Carbon itself is not harmful. What matters is where it is, what chemical form it takes, how much accumulates and how quickly it moves.[15]

Most of Earth’s carbon is stored in rocks and sediments. The rest is in the ocean, the atmosphere, soils and living organisms. These stores are called reservoirs. Carbon moves among them through photosynthesis, respiration, decomposition, ocean–atmosphere exchange, combustion, and weathering and other geological processes.[7][15]

Some exchanges are fast: plants take up carbon dioxide and release it again within days to years, and the surface ocean exchanges carbon dioxide with the air relatively quickly. Others are very slow: carbon can remain in the deep ocean for centuries and in rocks for millions of years.[7][15]

Burning coal, oil and natural gas rapidly moves carbon that was locked away over geological time into the active atmosphere–ocean–land system. Land-use change and cement production also add carbon dioxide. When additions exceed removals by land and ocean, carbon dioxide accumulates in the atmosphere.[15][16]

Level 1 · Understand the basics

The IPCC’s 2023 Synthesis Report concludes that human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming.[1]

IPCC AR6 assessed that global surface temperature during 2011–2020 was about 1.1 °C (1.09 °C) above the 1850–1900 baseline. That figure describes one decade compared with one baseline period. It is not the temperature anomaly for 2026.[1][16]

Warming changes more than temperature. It alters precipitation and the water cycle, raises sea level and influences heat extremes. These changes put pressure on ecosystems and biodiversity, on food and water systems, and on coastal communities and infrastructure.[1][14][16]

Outcomes are not the same everywhere. Local effects depend on geography, existing ecosystems, land use, infrastructure and the capacity of communities to adapt.[16]

Level 1 · Understand the basics

Net-zero emissions means that the emissions people add to the atmosphere are balanced by the emissions people remove, over a specified period. It does not mean that every human activity has zero emissions.[1][16]

The order matters: reduce emissions as deeply as feasible, avoid creating new emissions where possible, and use carbon dioxide removal to balance residual emissions that are hard to eliminate. Removals cannot simply substitute for reductions at any scale.[1]

Net-zero CO2 and net-zero greenhouse gas emissions are different targets. Net-zero CO2 balances carbon dioxide alone. Net-zero greenhouse gases also covers methane, nitrous oxide and other gases, which requires conversion metrics because gases differ in strength and lifetime. IPCC AR6 explains that net-zero CO2 would approximately stabilise CO2-induced warming, while net-zero greenhouse gas emissions would halt human-induced warming or allow a slight decline from its peak.[16]