Measurements of air temperature near the Earth’s surface have been made over a large enough area to infer global climate change since the mid-19th century. In the last 40 years, satellite measurements (or rather remote observations) have been added to the measurements made at the surface, both on land and at sea. Although the different measurement series differ in their measurement techniques, the degree of coverage of the Earth’s surface measurements and the ways in which the data are processed and analysed, they unanimously show that since the second half of the 19th century, the average surface temperature of our planet has increased by about 1°C, rising particularly rapidly in recent decades.

In addition to directly measured air temperatures at the Earth’s surface, many other observations attest to the progressive warming, such as shifting climate zones, the disappearance of Arctic sea ice, the melting of the Greenland and Antarctic ice sheets and glaciers, rising ocean levels, increasingly frequent heat waves, the warming of the oceans from the surface in depth, the thawing of the permafrost (once called ‘permafrost’), the migration of species towards the poles and to higher altitudes, and many other phenomena.

As a result of the Earth’s radiative imbalance (created by an increase in atmospheric concentrations of greenhouse gases), the entire planet and therefore its atmosphere, oceans, land and glaciers accumulate energy. To have a complete picture of global warming, you have to take into account all the energy changes in the system. We can study changes in the energy content of the atmosphere (we know its heat capacity and temperature changes), the energy used to melt glaciers and ice sheets (the decisive factor here is the amount of mass they lose, which translates into the amount of energy used for the phase transformation of ice into water; changes in the temperature of the ice are less important), the energy going into heating soils and going into the oceans.

Analyses show that 93% of all surplus energy accumulating in the Earth’s climate system ends up in the oceans, after approx. 3% goes to heating the ground on land and melting ice (sea ice, in glaciers and ice sheets), and only 1% to heating the atmosphere. Recall: the heat capacity of the oceans is more than a thousand times that of the atmospher Furthermore, heat from the surface of the oceans is very easily dissipated into the depths, unlike soils or ice, which are very effective insulators compared to water.

Thus, we observe that thermal energy in the oceans is increasing at a rate of 1.1-10²²J per year. If that doesn’t tell you much, don’t worry – it’s actually a very large number. Let’s convert it to something more tangible. If we heated up the oceans with the detonation of atomic bombs such as the one dropped on Hiroshima in 1945 (with a power of 15 kilotons of TNT equivalent), how often would we have to run such a “heater”? Converted into joules, the explosion energy of such a nuclear bomb is 6.3-10¹³ J. So we would have to detonate 1.1-10²² / 6.3-10¹³ = 175 million bombs per year (or nearly half a million per day).

The question naturally arises: is the current warming something unusual? And another: how does it relate to the climate change that was taking place earlier?

Thousands and millions of years ago, no one measured temperatures, greenhouse gas concentrations, sea levels or solar activity. Not only for lack of suitable measuring instruments – for most of Earth’s history there was also a lack of a suitably intelligent ‘someone’. In order to learn about the distant history of our planet, we therefore have to reach for the so-called climate proxies (proxy), i.e. data from which parameters describing the climate e.g. temperature, atmospheric composition or water availability) can be indirectly estimated. Traces of past climate change are available in many places, from ocean depths to ice sheets – you just need to know how to track them down and read them. Valuable information is provided by plant and animal remains, tree rings, cave dripstone, ice cores, ocean and lake sediments, etc.

Reaching further back into the past, we can see how stable the climate has been over the last 11,500 years, and we can also trace the Earth’s climate’s exit from an ice age state (which happened earlier).

You can see that climate change is something normal in the history of our planet – 20 000 years ago we had the maximum of the ice age, when northern Poland and large parts of America and Asia were covered by a huge ice cap. Climatic zones, deserts and the coastline were completely different (because much of the water was not in the oceans but in the ice sheets). Later, the Earth began to emerge from the Ice Age, and around 10,000 years ago, the Earth’s climate stabilised at its current level, with temperatures around 4°C higher than at the time of the Ice Age maximum (various reconstructions show a range of temperature changes from 3 to 5.5°C).

This epoch of not significantly changing temperatures is the Holocene – the heyday of our civilisation. The stable climate suited us very well – there was no shifting of climate zones, distribution of deserts or land suitable for agriculture. So we settled: we established villages and then towns. The coastline had also stabilised, so we could build harbour towns along it.

Ostatni wzrost temperatury, zaznaczony na ilustracji 5. czerwonym kolorem, wyraźnie odbiega charakterem od pozostałej części wykresu. Średnia globalna temperatura w dekadzie 2007–2016 przekraczała średnią z holocenu o ok. 1,2°C, a średnią temperaturę najcieplejszego stulecia holocenu o ponad 0,3°C. Oznacza to, że jest ona obecnie największa co najmniej od interglacjału eemskiego 125 tys. lat temu (źródło).

Artykuł powstał dzięki pomocy Marcina Popkiewicza