Vprašanje za geofizike in astronome: Kakšna je povezava med sedanjimi podnebnimi spremembami in spremembami naklona rotacijske osi zemlje? Sprašujem zaradi dveh znanstvenih odkritij.

Prvič, pred 4 dnevi je ugledna revija Nature objavila povzetek znanstvenega članka, ki ugotavlja, da je povečana aktivnost črpanja vode (predvsem v Indiji in zahodnem delu severne Amerike) s praznjenjem podzemnih rezrervoarjev spremenila težišče zemlje, povzročila pospešen premik severnega pola in spremenila naklon rotacijske osi zemlje. Natančneje:

Gravitational surveys have measured the depletion of underground reservoirs, which is caused in large part by irrigation, especially in northwestern India and western North America. These surveys show that groundwater pumping shifted enough mass into the oceans to cause 6.24 millimetres of global sea-level rise between 1993 and 2010.

By including these changes in their model, the authors calculated that they should have a substantial impact on the Earth’s rotation axis. They predicted that the displacement of groundwater alone causes a shift in the North Pole of 4.36 centimetres per year, roughly in the direction of Russia’s Novaya Zemlya islands.

In drugič, pred sto leti je srbski geofizik in astronom Milutin Milanković odkril, da je cikle v dolgoročnih klimatskih spremembah mogoče pojasniti s spremembami pozicije zemlje glede na sonce oziroma da so dolgoročni skupni učinki sprememb položaja zemlje glede na sonce močno gonilo dolgoročnega podnebja na Zemlji in so odgovorni za sprožitev začetka in konca ledeniških obdobij (ledene dobe). Milanković je preučeval tri vrste ciklov, povezanih s (1) obliko površja zemljine orbite, (2) naklonom rotacijske osi zemlje in (3) smerjo rotacijske osi zemlje. Največji učinek izmed navedenih treh na klimatske spremembe naj bi imel naklon rotacijske osi zemlje, ker najbolj vpliva na osončenost na določenih dolžinah zemlje.

Kot pravi dober povzetek na spletni strani Nase:

Specifically, he examined how variations in three types of Earth orbital movements affect how much solar radiation (known as insolation) reaches the top of Earth’s atmosphere as well as where the insolation reaches. These cyclical orbital movements, which became known as the Milankovitch cycles, cause variations of up to 25 percent in the amount of incoming insolation at Earth’s mid-latitudes

…

The small changes set in motion by Milankovitch cycles operate separately and together to influence Earth’s climate over very long timespans, leading to larger changes in our climate over tens of thousands to hundreds of thousands of years. Milankovitch combined the cycles to create a comprehensive mathematical model for calculating differences in solar radiation at various Earth latitudes along with corresponding surface temperatures. The model is sort of like a climate time machine: it can be run backward and forward to examine past and future climate conditions.

Milankovitch assumed changes in radiation at some latitudes and in some seasons are more important than others to the growth and retreat of ice sheets. In addition, it was his belief that obliquity was the most important of the three cycles for climate, because it affects the amount of insolation in Earth’s northern high-latitude regions during summer (the relative role of precession versus obliquity is still a matter of scientific study).

He calculated that Ice Ages occur approximately every 41,000 years. Subsequent research confirms that they did occur at 41,000-year intervals between one and three million years ago. But about 800,000 years ago, the cycle of Ice Ages lengthened to 100,000 years, matching Earth’s eccentricity cycle. While various theories have been proposed to explain this transition, scientists do not yet have a clear answer.

Milankovitch’s work was supported by other researchers of his time, and he authored numerous publications on his hypothesis. But it wasn’t until about 10 years after his death in 1958 that the global science community began to take serious notice of his theory. In 1976, a study in the journal Science by Hays et al. using deep-sea sediment cores found that Milankovitch cycles correspond with periods of major climate change over the past 450,000 years, with Ice Ages occurring when Earth was undergoing different stages of orbital variation.

Several other projects and studies have also upheld the validity of Milankovitch’s work, including research using data from ice cores in Greenland and Antarctica that has provided strong evidence of Milankovitch cycles going back many hundreds of thousands of years. In addition, his work has been embraced by the National Research Council of the U.S. National Academy of Sciences.

Scientific research to better understand the mechanisms that cause changes in Earth’s rotation and how specifically Milankovitch cycles combine to affect climate is ongoing. But the theory that they drive the timing of glacial-interglacial cycles is well accepted.

Torej, logično vprašanje od nekoga, ki ni geofizik, pri čemer se že vnaprej opravičujem, če je vprašanje butasto:

• Ali in v kolikšni meri je sedanje globalno segrevanje mogoče pojasniti z navedenimi odkritimi povečanimi spremembami rotacijske osi zemlje (zaradi človeške dejavnosti povečanega črpanja vode)? So te spremembe naklona dovolj velike, da bi lahko imele pomemben vpliv na sedanje globalno segrevanje?
  
• In naprej, glede na to, da se dolgoročna nihanja vsebnosti CO₂ skladajo z nihanji temperature, ali je možno, da so sedanje povečane vsebnosti CO₂ posledica porasta temperature, do katerega je prišlo zaradi spremembe rotacijske osi zemlje?