Climate Change and its Causes

Year 12 AS Geography

Is global warming a recent short term phenomenon or should it been seen as

part of long term change?

Learning Objectives:• To examine the evidence for long, medium

and short term climate change;• To explore the natural and human causes of

climate change;• To assess the evidence for and against the

current view that climate change is unprecedented and largely the result of human activity

What is climate change?

• Climate change is any long term change or shift in climate detected by a sustained shift in the average value for any climatic element (e.g. rainfall, drought, storminess)

• It is very important to remember that climate change can be assessed at a range of scales: short, medium and long and different pieces of evidence are used to measure and record climatic changes.

Long term climate change Evidence

• Measured on the geological timescale over several hundreds of thousands to millions of years. Evidence for this mainly comes from ice core data.

Medium term climate change• Proxy records are mainly used to reconstruct climate change

before the start of instrumental records. These include paintings, poems, books, diaries and journals which document the weather at this time

This photo depicts the freezing of the River Thames, London in 1677.

Global warming? Recent climate change

The Instrumental Record• Used for the last 100 years or so• They show that near surface air

temperatures have risen by 0.74 degrees between 1900 and 2000.

• The warming trend has been near constant since 1960 and 11 of the 12 hottest years have occurred in the decade 1995 – 2006

• Ocean temperatures have warmed to depths of 3000m and the pH of the oceans has decreased, most likely due to increased levels of dissolved CO2.

• Global sea levels have also risen. This is mainly attributed to thermal expansion, with water from melting glaciers and ice caps having less of an impact.

Ice Response• Ice is a key indicator

for climate scientists as in a warming world much of the sea ice and glaciers would be expected to melt.

Natural causes of climate change

• Key term: Climate forcing: any mechanism that alters the global energy balance and forces the climate to change in response.Climate feedback effects are those that can either amplify a small change and make it larger (positive feedback) or diminish the change and make it smaller (negative feedback)