Mauna Loa Observatory. Public domain.

How do we know that CO2 concentrations are highest in more than 300,000 years?

Apr 28 2025

As the latest data on carbon dioxide concentrations hits the headlines, it's a good time to dive into how we measure these concentrations - and why they matter.

By Jed Thomas

High on the desolate slopes of Mauna Loa, an active volcano on Hawaii’s Big Island, sits one of Earth’s most important scientific outposts.

The Mauna Loa Observatory (MLO), at 3,397 meters (11,135 feet) above sea level, has quietly recorded the atmosphere’s changing chemistry for nearly seven decades.

It was here, in 1958, that scientist Charles David Keeling began measuring carbon dioxide (CO₂) concentrations, producing what would become the iconic Keeling Curve, the clearest and most enduring evidence of the rise in greenhouse gases.

What is the Mauna Loa observatory?

MLO was established by the U.S. Weather Bureau in 1956, specifically to monitor the free atmosphere, beyond the immediate influences of vegetation, urban pollution, or localized weather.

Although the summit of Mauna Loa was initially considered, practical challenges led to the observatory’s placement slightly downslope, where conditions were still ideal for capturing global atmospheric trends

The site’s advantages are manifold. Its remote location in the Pacific, thousands of miles from major industrial centres, allows it to sample air that is broadly representative of the northern hemisphere.

Sitting above the marine inversion layer, the observatory has access to clean, well-mixed air, largely free from local pollution.

Moreover, the volcanic landscape’s sparse vegetation minimises seasonal swings caused by photosynthesis and decay, while the regular diurnal wind patterns help maintain consistency in air quality.

Despite being located on an active volcano, MLO’s CO₂ readings are carefully protected from contamination.

Local volcanic emissions are rare at the observatory and easily identifiable: when they occur, they cause sharp, sudden spikes in CO₂ levels that differ dramatically from the typical seasonal fluctuations.

Such data points are flagged and excluded from the final record.

Cross-referencing with other observatories worldwide and with NASA’s satellite measurements confirms that Mauna Loa’s long-term trend mirrors the broader global pattern.

How do we know that UK GHG emissions are really falling?

Beyond the headlines, emissions monitoring professionals must learn (and continually update) methodologies to assign emissions more accurately. By Jed ThomasThe UK’s territorial greenhouse gas (... Read More

How does Mauna Loa measure CO2 concentrations?

Inside the observatory, CO₂ levels are measured using high-precision infrared gas analysers.

Air is drawn continuously into the instruments, dried to remove water vapor, and analysed for CO₂ concentration, expressed in parts per million (ppm).

Calibration against carefully maintained reference gases ensures that measurements are consistent over decades.

Only hourly data that meet strict background conditions are used to generate daily, monthly, and yearly averages.

How do climatologists know past CO₂ concentrations?

While Mauna Loa provides the longest direct record of CO₂ measurements, understanding how today’s concentrations compare to ancient periods relies on entirely different methods.

Scientists reconstruct past atmospheric CO₂ levels using ice core records, cylinders of ice drilled from ice sheets in Greenland and Antarctica.

As snow fell and compacted into ice over millennia, it trapped tiny bubbles of air.

By analysing the gases sealed inside these bubbles, researchers can measure the chemical composition of Earth’s atmosphere stretching back over 800,000 years.

These ice core records show that for at least 300,000 years before the Industrial Revolution, CO₂ concentrations fluctuated between about 180 and 280 ppm, closely tracking natural glacial and interglacial cycles.

The sudden rise past 400 ppm in the last century is unprecedented in this natural history.

For even earlier periods, such as the Pliocene Epoch (about 3 to 5 million years ago), scientists rely on proxy data, indirect evidence from sources like ocean sediments, fossilised plant matter, and isotopic analyses of ancient soils and rocks.

These methods are less precise than ice cores but offer a clear picture: during the Pliocene, when CO₂ levels last matched today’s concentrations, global temperatures were significantly higher, sea levels were tens of meters higher, and ecosystems were profoundly different.

The latest data: record CO₂ increases

The latest readings from Mauna Loa tell a sobering story.

In 2024, atmospheric CO₂ levels rose by 3.58 ppm, the largest single-year jump recorded since continuous observations began.

By March 2025, the monthly average concentration had climbed to 428.15 ppm, up from 425.38 ppm a year earlier.

Several factors contributed to this unprecedented surge:

Record levels of fossil fuel combustion and deforestation in 2024.

Widespread wildfires, especially across the Northern Hemisphere, driven by record-breaking heat and exacerbated by the El Niño climate pattern.

Potential weakening of natural carbon sinks like forests and oceans, which normally absorb about half of anthropogenic CO₂ emissions.

Wildfires: do we really know how much CO2 they release?

More wildfires means more carbon emissions - but do we know how much more?By Jed ThomasWildfires have intensified globally, becoming more frequent, extensive, and severe, driven by climate change a... Read More

These figures underscore an accelerating trend that increasingly undermines hopes of limiting global warming to 1.5°C above preindustrial levels.

The gravity of the moment was captured succinctly by Dr. Genevieve Guenther, a climate expert affiliated with The New School’s Tishman Environment and Design Center and an Expert Reviewer for the UN’s Intergovernmental Panel on Climate Change. Reflecting on the data, Dr. Guenther wrote on X:

“The last time atmospheric CO₂ was this high, 3-5 million years ago, the oceans ended about one hundred miles west of what would become the coast of North America, East Africa was heavily forested, and there were camels in what is now the Arctic. Humans did not yet exist.”

Why Mauna Loa remains vital

While satellites now provide global coverage of CO₂ concentrations, the Mauna Loa Observatory remains the gold standard for long-term atmospheric monitoring.

Its uninterrupted, precisely calibrated record since 1958 serves not only as an archive of humanity’s impact on the atmosphere but also as a critical benchmark against which other measurements are compared.

Each new measurement from MLO deepens our understanding of the urgency of climate action.

With every passing year and every added part per million, the instruments atop Mauna Loa continue their vital work, bearing witness to a world in rapid transformation.