Human Influence

Before the Industrial Revolution (around 1750), the concentration of CO₂ in the atmosphere was about 280 ppm (parts per million). It has now risen above 420 ppm, higher than at any time in at least 800,000 years. Two human activities are mainly responsible.Burning fossil fuels (coal, oil and gas) for electricity, transport and industry releases carbon that was locked away for millions of years. This is by far the largest human source of CO₂.Deforestation adds CO₂ in two ways: burning or decay of the cleared trees releases their stored carbon, and fewer trees remain to remove CO₂ by photosynthesis.Measurements at Mauna Loa in Hawaii since 1958 (the Keeling curve) show a steady rise in CO₂ with a small yearly fluctuation. CO₂ falls each northern summer, when photosynthesis in the large land area of the Northern Hemisphere exceeds respiration, and rises each winter.About half of the CO₂ humans emit stays in the atmosphere; the rest is taken up by the oceans and land vegetation. Humans can reduce their effect by using renewable energy, planting trees (reforestation) and using carbon neutral biofuels, which release only the CO₂ the crop absorbed as it grew.Worked example: CO₂ rose from 316 ppm in 1959 to 420 ppm in 2024. The mean rate of increase is:(420 − 316) ÷ 65 = 1.6 ppm per year