Weather and climate
Weather is the state of the air at one place and time: hot, rainy, windy. It changes by the hour. Climate is the average weather of a place over a long time, usually 30 years or more.
The climate system includes the air (atmosphere), oceans, ice, land and living things. They all swap heat and water. Things that affect a place's climate include distance from the equator (latitude), height above the sea, nearness to the sea, winds and ocean currents.
Climate change means a long-lasting change in these averages. Earth's climate has changed naturally in the past (ice ages), but today's change is fast and mostly caused by humans.
The greenhouse effect
- Sunlight passes through the air and warms the land and sea.
- The warm surface gives out heat as infrared radiation.
- Greenhouse gases absorb some of this infrared and send part of it back down.
The main greenhouse gases are water vapour, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and some man-made gases (CFCs, HFCs). Nitrogen and oxygen, which make up most of the air, are not greenhouse gases.
The natural greenhouse effect is good: without it Earth's average would be about −18 °C. The problem is the enhanced greenhouse effect caused by extra gases.
Human causes and the evidence
- Burning fossil fuels (coal, oil, natural gas) for power, factories and transport releases CO₂.
- Deforestation: fewer trees take in CO₂, and burning forests releases it.
- Farming: cattle and rice fields give off methane; fertilisers give nitrous oxide.
- Cement and waste: cement making releases CO₂; landfills give methane.
How do we know?
- Air bubbles trapped in ice cores show CO₂ was about 280 ppm before 1750; it is now above 420 ppm (ppm = parts per million).
- Thermometer records show the world is about 1.2 °C warmer than in the late 1800s.
- Glaciers are shrinking, Arctic sea ice is thinning, and sea level has risen about 20 cm since 1900.
- Tree rings, corals and lake mud show past climates for comparison.
Effects and feedback loops
- Melting ice and rising seas: low coasts and islands flood more often.
- Extreme weather: stronger heatwaves, heavier rain and floods, longer droughts, more wildfires.
- Oceans: warmer water bleaches coral; extra CO₂ makes seawater more acidic.
- Living things: plants and animals move towards the poles or up mountains; some cannot move fast enough.
- People: harm to crops, water supply and health; the poorest often suffer most.
Feedback loops
A positive feedback makes warming bigger. Example: ice melts → darker sea or land absorbs more sunlight → more warming → more ice melts. Thawing frozen ground (permafrost) releasing methane is another.
Mitigation and adaptation
Mitigation means cutting greenhouse gases: solar, wind and hydro power; electric and public transport; saving energy at home; protecting and planting forests; less food waste.
Adaptation means preparing for the changes already coming: sea walls and mangroves along coasts, early warning for storms and heat, water harvesting, and crops that survive drought.
Countries work together through the United Nations. The Paris Agreement (2015) aims to keep warming well below 2 °C and try for 1.5 °C. India's National Solar Mission and many cities' heat action plans are examples of action.
Try it: a jar greenhouse
Put two thermometers in the sun. Cover one with a clear glass jar. Read both every 5 minutes for 30 minutes and draw a line graph. The one under the jar gets warmer because heat cannot escape easily. This is a simple model of a greenhouse. Then use the CO₂ slider in the 3D and predict before you slide.
Key formulas and definitions
- Key term: Climate = average weather over about 30 years
- Key term: Greenhouse gases = water vapour, CO₂, CH₄, N₂O, CFCs
- Key term: Mitigation = cutting the cause; Adaptation = coping with the effects
- Rough rule: doubling CO₂ gives about 3 °C of long-term warming
Worked examples
1. Is "It rained heavily in Mumbai yesterday" about weather or climate?
Weather. It describes one day. "Mumbai gets most of its rain from June to September" would be climate.
2. CO₂ rose from 280 ppm to 420 ppm. By how many ppm and by what percentage?
Rise = 420 − 280 = 140 ppm. Percentage = 140 ÷ 280 × 100 = 50%.
3. Classify each as mitigation or adaptation: (a) solar panels, (b) raising houses on stilts in a flood zone.
(a) Mitigation, it cuts emissions. (b) Adaptation, it protects people from floods.
4. Why does melting Arctic ice speed up warming?
White ice reflects sunlight. When it melts, dark sea water absorbs more sunlight and heats up, which melts more ice. This is a positive feedback loop.
Common mistakes
- Mixing up weather and climate. One cold day does not disprove climate change.
- Thinking the ozone hole causes global warming. They are different problems.
- Thinking the greenhouse effect is bad in itself. The natural effect keeps Earth liveable; the extra (enhanced) effect is the problem.
- Saying melting sea ice is the main cause of sea level rise. Land ice melting and warm water expanding are the main causes.