Would the climate be warming without humans?
Image: Joseph Chan
Causes
- A brief history of climate change: who knew what, when
- What causes climate change?
- Would the climate be warming without humans?
- Is it just a cycle? (Earth’s wobbly orbit)
- Sunspots & solar activity
- Land use: agriculture & cities
- Volcanoes
- Ocean currents
- Black carbon & ash
- Albedo effect
- Hydrogen
- Greenhouse gases & how they work
- – Carbon dioxide & the carbon cycle
- – Methane: biogenic (mostly cows) & ‘natural’ gas
- – Nitrous oxide (mostly agriculture)
- – Clouds & water vapour
- – Ozone
- – Man-made industrial chemicals
- – Aerosol pollution
- How to start an Ice Age!
- What’s in a name?
Home > Climate wiki > What causes climate change? > Humans
Would the climate be warming without humans?
Summary
In the 4.54 billion years since Earth formed, natural climate factors or forcings have changed Earth’s climate, making it warmer or cooler many times, for different reasons outlined in the menu on this page.
The nice weather we’ve had for the last 4,500 years allowed us to build a global civilization. Ironically, burning fossil fuels and eradicating natural ecosystems that helped keep the carbon cycle in balance, has dangerously destabilised the climate. Today, when all of the natural forcings are added together, Earth should be cooling. Instead, we’re warming rapidly (Figs. 1 & 2).
We know from fundamental laws of physics that greenhouse gases trap heat in the atmosphere, and why that changes the chemistry of the atmosphere making it warmer. We also know we humans are emitting lots of these greenhouse gases (Video 2). No matter how much we might wish otherwise, there is no negotiating with the laws of physics and chemistry: we humans are entirely responsible for turning the heat nob on Earth’s thermostat to ‘high’.
Any opinion to the contrary would need to explain why global temperatures are rising instead of falling, and explain why fundamental laws of physics and chemistry have stopped working.
The good news is that we can use nature to capture and store excess greenhouse gases back in the ground, so we can limit the impacts.
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Composite graph from Carbon Brief shows that an increase in temperature from greenhouses gases (red line) matches the warming we’ve observed (light grey line). Each of the natural and anthropogenic cooling and warming ‘forcings’ are explained in WHAT CAUSES CLIMATE CHANGE (menu at the top left).
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Fig. 2: The blue line shows that all the natural climate forcings combined are very slowing cooling the planet. The red line shows that human factors far outweigh this cooling, even when volcanic eruptions lead to short drops in temperature. So, on balance, the Earth is warming rapidly. NOTE: not all areas of the planet are warming equally. The Arctic and Antarctic are warming more than 4 times than the rest of the world due to the Albedo Effect. (Image: Carbon Brief)
Causes
- A brief history of climate change: who knew what, when
- What causes climate change?
- Would the climate be warming without humans?
- Is it just a cycle? (Earth’s wobbly orbit)
- Sunspots & solar activity
- Land use: agriculture & cities
- Volcanoes
- Ocean currents
- Black carbon & ash
- Albedo effect
- Hydrogen
- Greenhouse gases & how they work
- – Carbon dioxide & the carbon cycle
- – Methane: biogenic (mostly cows) & ‘natural’ gas
- – Nitrous oxide (mostly agriculture)
- – Clouds & water vapour
- – Ozone
- – Man-made industrial chemicals
- – Aerosol pollution
- How to start an Ice Age!
- What’s in a name?
Home > Climate wiki > What causes climate change? > Humans
Summary
In the 4.54 billion years since Earth formed, natural climate factors or forcings have changed Earth’s climate, making it warmer or cooler many times, for different reasons outlined in the menu on this page.
The nice weather we’ve had for the last 4,500 years allowed us to build a global civilization. Ironically, burning fossil fuels and eradicating natural ecosystems that helped keep the carbon cycle in balance, has dangerously destabilised the climate. Today, when all of the natural forcings are added together, Earth should be cooling. Instead, we’re warming rapidly (Figs. 1 & 2).
We know from fundamental laws of physics that greenhouse gases trap heat in the atmosphere, and why that changes the chemistry of the atmosphere making it warmer. We also know we humans are emitting lots of these greenhouse gases (Video 2). No matter how much we might wish otherwise, there is no negotiating with the laws of physics and chemistry: we humans are entirely responsible for turning the heat nob on Earth’s thermostat to ‘high’.
Any opinion to the contrary would need to explain why global temperatures are rising instead of falling, and explain why fundamental laws of physics and chemistry have stopped working.
The good news is that we can use nature to capture and store excess greenhouse gases back in the ground, so we can limit the impacts.
-
Composite graph from Carbon Brief shows that an increase in temperature from greenhouses gases (red line) matches the warming we’ve observed (light grey line). Each of the natural and anthropogenic cooling and warming ‘forcings’ are explained in WHAT CAUSES CLIMATE CHANGE (menu at the top left).
-
Fig. 2: The blue line shows that all the natural climate forcings combined are very slowing cooling the planet. The red line shows that human factors far outweigh this cooling, even when volcanic eruptions lead to short drops in temperature. So, on balance, the Earth is warming rapidly. NOTE: not all areas of the planet are warming equally. The Arctic and Antarctic are warming more than 4 times than the rest of the world due to the Albedo Effect. (Image: Carbon Brief)
More information
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The Holcene Epoch—the geological epoch we are now living in—began at the end of the Pleistocene Glacial Epoch ~11,650 years ago after most of the glaciers and ice sheets had melted. There were a few climate blips for the next 7,000 years, but from about 4,500 or so years ago until recently, the global climate was particularly stable.
This was a period when agriculture became well-established and civilisation spread across the globe.
The amount of greenhouse gases in the atmosphere very slowly crept up because of agriculture, but the uneven orbit of the Earth around the sun was very very slowly leading is back into another Glacial Epoch. The very gradual cooling was offsetting the warming causes by agriculture. Consequently, on balance, the global average temperature remained fairly constant.
The so-called ‘Little Ice Age’ (next tab) and ‘Mediaeval Warm Period‘ were tiny climate blips that didn’t last long (geologically speaking). The first was triggered by volcanic eruptions and was largely felt in the Northern Hemisphere. Climate proxy records show that these events didn’t have the same impact across the globe.
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This is generally regarded as the period 1300 – 1850AD. Several forcings appeared to have contributed, including the 1257 eruption of Samalas volcano. The AMOC (Atlantic oceanic current) also seems to have slowed due to a sudden influx of freshwater following the ‘Medieval Warm Period’ 950-1250AD. A part of this current, the Gulf Stream, is crucial for keeping Europe warm. Slight changes have global impacts, which is why it’s a concern today, as the current is slowing down.
A very small contribution to the cooling may have been from a slight decline in solar radiation. However, the Maunder Minimum didn’t begin until 1645AD (Fig. 4) three hundred years after cooling began in the Northern Hemisphere.
A very small contribution may have been from a slight decline solar radiation. However, the Maunder Minimum didn’t begin until 1645AD (Fig. 4; see Sunspots and Solar activity), three hundred years after cooling began in the Northern Hemisphere.
Actual ice ages: we’re currently living in the Quaternary Period, an Ice Age. Confused? Don’t be. This page explains the difference between ice ages (Periods) and glacials (epochs).
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Radiative Forcing or RF is due to the Law of Conservation of Energy, a basic law of thermodynamics, which states that:
‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’
The Stefan-Boltzmann Law explains how natural greenhouse gasses (natural forcings) keep the Earth’s surface ~33°C warmer than it would be without them. Without them, we would be a frozen snowball. So, we do need greenhouse gases, but the balance needs to be just right.
Different climate forcings each determine how much solar energy arrives and escapes.
- Natural Forcings are those that happen through natural changes.
- Anthropogenic Forcings are those due to human activities.
The Clausius-Clapeyron Equation describes a discontinuous phase transition between the different states (gas, liquid, solid) of water.
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In Video 1, Professor Richard Alley explains how the chemistry of the atmosphere clearly points to humans burning fossils fuels.
Here in Aotearoa Grass cuttings from around the country are used to measure the proportion of carbon isotopes in New Zealand: Grass and the science of CO2 Radio NZ Podcast.
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“Nature across most of the globe has now been significantly altered by multiple human drivers, with the great majority of indicators of ecosystems and biodiversity showing rapid decline.
“Seventy-five per cent of the land surface is significantly altered, 66 per cent of the ocean area is experiencing increasing cumulative impacts, and over 85 per cent of wetlands (area) has been lost.”
– Diaz et al, 2019 Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services
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Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space. The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it much faster than the snow and ice (Fig. 3).As more ice forms, the water is cooler, leading to more ice forming, and so on, in a feedback effect. However:
Recent global temperature surge intensified by record-low planetary albedo – Science, 05 Dec. 2024 (Figs. 4 & 5)
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Image: Nathan Kurtz / NASA
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Image: Duspayev et al; Earth’s Sea Ice Radiative Effect from 1980 to 2023
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Image: Eliot Jaboson; data: NASA
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- 2025: Quilcaille et al; Systematic attribution of heatwaves to the emissions of carbon majors, Nature (Open access) 645 pp392-398
- 2025: Sadai et al; Estimating the sea level rise responsibility of industrial carbon producers, Environmental Research Letters 20 | 4 (Open access)
- 2024: Duspayev et al; Earth’s Sea Ice Radiative Effect from 1980 to 2023, Geophysical Research Letters 51 | 14 (Open access)
- 2024: Goessling et al; Recent global temperature surge intensified by record-low planetary albedo, Science Research Article 387 | 6729 pp68-73
- 2022: IPCC Sixth Assessment Report; Climate Change 2022: Impacts, Adaptation and Vulnerability
- 2021: IPCC Sixth Assessment Report; Climate Change 2021: The Physical Science
- New Zealand Climate Change Commission
- Carbon Brief Explainer: How the rise and fall of CO2 levels influenced the ice ages
- Carbon Brief: Why scientists think 100% of global warming is due to humans
- Carbon update: Global Action Tracker
- Science Direct: The Law of Conservation of Energy
- NOAA: Climate forcing
- NOAA: Ocean heat uptake
- IPPC: Chapter 8: Anthropogenic and Natural Radiative Forcing in:
Climate Change 2013: The Physical Science Basis. Contribution of
Working Group I to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change - NCIDSC (National Snow and Ice Data Centre): Climate change in the Arctic
- 2019: Diaz et al; Summary for policymakers of the global assessment report on biodiversity and ecosystem services, Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services.