Quick Infos

The UN’s Intergovernmental Panel on Climate Change (IPCC) kicked off its 2021 report with the following statement: “It is unequivocal that human influence has warmed the atmosphere, ocean and land.” 

How are we so certain? It took a while but climate modelling is now refined enough to predict how things would go without human influence, within a margin of error. What we are observing today, however, is beyond that margin of error, therefore proving that we have driven the change. (See also the right figure below.)

Most straightforward of our climate change facts: according to the IPCC’s sixth assessment report on the state of our climate, the past decade is likely to have been the hottest period in the last 125,000 years. For about 100,000 years, we have been oscillating between glacial (ice ages) and warmer interglacial periods like the one we currently live in. Yet, this is also the warmest multi-century period we have had in this timespan. 

The vertical bar on the left shows the estimated temperature (very likely range) during the warmest multi-century period in the last 100,000 years, which occurred around 6,500 years ago during our current era called the Holocene. Only around 125,000 years ago, a time prior to the last ice age, might have had higher temperature than the ones we are currently experiencing. Each of these past warm periods were caused by slow (multi-millennial) orbital variations that are not in play today. 

Pre-industrial CO2 levels were around 280 parts per million (ppm). Today, we stand close to 420 ppm. Not shown here is that that the CO2 shows a close correlation with the average global temperature.

The most distant period in time for which we have estimated CO2 levels is around the Ordovician period, 500 million years ago. Also, note that from 10'00 year ago until we started to change the climate, temperatures and CO-levels were fairly constant.

And this despite the fact that the ocean already comes to our rescue by absorbing about one-third of the carbon in the atmosphere. Before the industrial revolution, it was actually a source of carbon, and not a sink, but the massive amount of CO2 now in the atmosphere has forced it to start absorbing the gas. 

When I was a child, we had to clean the windshield at every other gas-stop, since there were that many dead bugs on it. Nowadays, you can drive for hundreds of kilometers, without killing a single insect. The chilling numbers:

• In Europe, decades of monitoring in nature reserves have shown a loss of over 75% in flying insects. North America has also shown similar patterns. Broader trends still suggest that many tropical insect populations, such as those in Costa Rica, are experiencing significant declines, and the vanishing of species is now reaching the next level of animals in the food chain.

• The massive conversion of forests, wetlands, grasslands, and other terrestrial ecosystems has produced a 60 percent decline (on average) in the number of vertebrates worldwide since 1970, with the greatest losses in vertebrate populations occurring in freshwater habitats (83 percent).

• According to the World Bank, almost 90% of global marine fish stocks are fully exploited or overfished. And over the past 40 years, marine species have seen a decrease of 39%.

A report from The Lancet found that the number of work hours lost to heat across the globe increased from 199 billion in 2000 to 295 billion in 2020. That is equivalent to 88 work hours per employed person. And an estimated 9 million people per year are killed by air pollution.

A report by the global reinsurance company Swiss Re from April 2025 estimates that the total losses from natural catastrophes, including those not covered by insurance, came in at $318 billion in 2024, the report said. That is up from $292 billion in 2023 and significantly above longer-term averages.

I want to close that article with a few numbers from my home country, Austria:

• While the global temperature has gone up about 1.5 °C from preindustrial time, the average temperature in Austria has already increased by 3.1 °C. In addition, until the end of this century this temperature will go up again by at least a further 2 °C.

• Since 1850 the total glacier mass has been reduced by approximately two-thirds. By 2050, only about one third of the current glacier mass will still exist. In other words, by 2050 only about 10% of the Austrian glaciers will still exist. As a consequence, summers will become significantly dried, also affecting farming and agriculture.

• The number of very hot days (> 30 °C) in Vienna has increased from an average of approximately 10 days a year to currently almost 40 days.