World leaders are meeting for the 29th time to address the causes and effects of climate change. These five questions and answers show how much our planet has already changed.

#1 Which regions emit the most CO2?

Governments are increasingly pledging shifts towards carbon neutral economies within the next 10 to 30 years. With emissions stabilizing in Europe and the Americas yet rising in Asia and Africa, the following chart underscores the  economic turnaround necessary to achieve carbon neutrality.

Absolute emissions, however, only tell half the story. Countries in Asia have seen immense population growth over the last decades, and more people leads to greater consumption of resources.

So seen from a CO2 per capita perspective, the picture is very different. Factoring in population size puts the spotlight on both Western countries like the US and Australia, as well as nations in other parts of the world, including Russia, Saudi Arabia, Oman, Qatar and Mongolia.

In the discussion about who should contribute most to emissions reductions, experts argue that not all nations can be held equally responsible, and that economic power and wealth should be taken into account.

Clustering countries in terms of income groups (see below) shows a connection between higher levels of income and higher median emissions per capita. It also reveals how countries within each group vary widely, and that the higher the income group, the wider the spread across the emissions spectrum.

High income countries with elevated emissions, such as global top-emitter Qatar, spew out much more CO2 per capita than countries like Germany and France, although they are in the same income group.

And while countries like India and China rank low on per capita emissions, their decisions still have a big impact, given their huge population numbers (bubble size).

#2 What are the major sources of greenhouse gas emissions?

At 28%, the power sector accounts for the greatest source of GHG emissions. 

Given the correlation between economic strength and CO2 emissions, the industrial sector is responsible for one of the biggest shares (22%) of overall greenhouse gases (GHG) — including methane and nitrous oxide — released into the atmosphere.

At 20%, agriculture, forestry and change in land use collectively account for the second greatest source of GHG emissions.

Over the last two decades, the annual amount of tree cover lost has gradually increased. Russia, Brazil and Canada were the world's biggest drivers of deforestation in 2023. The big increase in Canada compared to 2022 was due to the extensive wildfires destroying large parts of the country’s forests.

Deforestation is not only problematic because the CO2 previously stored in the ground and the trees themselves is released into the atmosphere, but also because forests and soils are "carbon sinks" that absorb atmospheric CO2 — making them a valuable tool in the fight against climate change.

#3 How have CO2 emissions developed over the last centuries?

CO2 emissions from burning fossil fuels have been on the rise since the early days of industrialization. However, as humans produced higher levels of carbon dioxide, Earth absorbed it in natural "carbon sinks," such as forests and oceans.

But as humanity began to produce more CO2 and other greenhouse gases than the planet's ecosystem could naturally absorb, more of those emissions became trapped in the atmosphere (red area in the following chart).

#4 How much has the world warmed already?

An increasing volume of CO2 particles traps the sunlight's warmth in the atmosphere, acting like a greenhouse in which it gets warmer and warmer. Compared to the 20th century — and the past five years in particular — the average global temperature has increased by 1.2 degrees Celsius (2.6 degrees Fahrenheit).

This change is measured by calculating the difference between temperatures observed at a specific time and place and the historical average for that same spot. The increase in temperature is the global average of those variations. The difference can be much greater on a local level or for individual months. For example, September 2024 was almost 2 degrees warmer than September 1956.

Such increases push Earth's temperature anomaly up and have potentially wide-ranging effects — from pockets of impossible heat to failing harvests and an increase in devastating extreme weather events such as drought, storms and floods. A rise in sea levels is among the most noticeable impacts as higher temperatures melt ice caps and glaciers and increase the total volume of water in the oceans.

#5 How much sea level rise do we already have?

According to data compiled by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Australian government scientific research agency, sea levels have risen nearly 25 centimeters (9.8 inches) in the last 140 years. Around one third of that increase happened in the last 25 years alone.

Sea levels are rising worldwide, but the trend is accentuated in the Arctic, which is heating faster than other regions.

The thermal property of water which allows it to expand when warmer, is also contributing to rising sea levels.

As usual, there is a caveat to the absolute numbers. While most of the world's oceans and seas are indeed higher than their historical levels, some areas are more affected than others.

Tide gauges in western Canada and northern Chile, for example, detect steady or even receding seas, whereas island countries in the southern Pacific and Indian oceans are witnessing alarming increases in levels — threats that could lead to them literally disappear beneath the waves.

Edited by: Anke Rasper and Tamsin Walker

This is an updated version of an article originally published during COP26 in 2021.