Risks of weakening of the Earth’s magnetic field
Paul Somerville, Chief Geoscientist, Risk Frontiers
The Earth’s magnetic field protects life from cosmic rays, energetic particles that would otherwise arrive from space. Mars now lacks a strong magnetic field, and the conditions on its surface are thought to be so damaging to life that any microbes that might inhabit the planet are thought to live safely beneath the surface. On Earth, the magnetic field ensures that life can flourish on the surface. However, as the Earth’s magnetic field weakens, its satellites start to fail. First, our communications satellites in the highest orbits go down. Next, astronauts in low-Earth orbit can no longer phone home. Eventually, cosmic rays start to bombard every living thing on Earth.
This is an eventuality that we are facing now and in the few next hundred years. If Earth’s magnetic field were to decay significantly, it could collapse altogether and reverse polarity from magnetic north – south to south – north, although Brown et al. (2018) do not think that a polarity reversal is likely. The consequences of this process could be dire for our planet, even if it does not lead to a reversal (Olson and Amit, 2006). The strength of the geomagnetic field has been decaying for the last 3,000 years (Olson and Amit, 2006), and at the current rate of decay it may be at a critically low level within a thousand years.
The Earth’s magnetic field is predominantly created by the flow of liquid iron inside the core. It has always been a feature of our planet, but it has reversed in polarity repeatedly throughout Earth’s history. Each time it reverses – up to 100 times in the past 20 million years, with the reversal taking about 1,000 years to complete – it leaves fossilised magnetisation in rocks and sediments on Earth that can be discovered by carbon dating of the mud in cores of sediments from the sea floor, and most recently, in the fibres of dead Kauri trees that have been preserved in swamps. For a polarity reversal to occur, the magnetic field needs to weaken by about 90% to a threshold level. As it weakens, the lack of a protective magnetic shield around our planet (Figure 1) allows more cosmic rays (high-energy particles from elsewhere in the universe) to generate cosmogenic isotopes such as carbon-14 whose radioactive decay rate can show when polarity reversals took place.
The last major reversal occurred between 772,000 and 774,000 years ago. Since then, the field has almost reversed 15 times, dropping in strength significantly in an excursion but not quite reaching the threshold needed to reverse before rising again. This is when we are most at risk – as the field decays and then recovers its strength. The last excursion occurred 42,000 years ago in an event named the Laschamp Excursion in which the poles briefly reversed, and evidence suggests we are heading for a weakening and possible reversal again.
The Laschamp Excursion and Adams Transitional Geomagnetic Event
The precise chronology of Laschamp Excursion has now been derived by linking two data sets. Firstly, measurements of the Earth’s magnetic field from sediment cores of the Black Sea by Nowaczyk et al. (2012) were matched with Greenland ice cores via climate variation documented at the same time. Secondly, the precise analysis and dating of the events was made possible by the radiocarbon (14C) analysis by Cooper et al. (2021) of a sub-fossil Kauri tree (like the one shown in Figure 3) that grew in the wetlands of Ngawha in northern New Zealand for about 1700 years during that time and was subsequently very well preserved in the swamps.
Not only did the magnetic field reverse polarity 42,000 years ago, it, first, dramatically lost strength. The Kauri tree ring record indicated that the magnetic field began to drop at 42,350 years ago and reached its lowest level 41,800 years ago, 300 years before the actual temporary pole reversal. The magnetic north pole moved south over a period of about 500 years as the magnetic field weakened to about 6% of its normal value. Over a period of about 500 years, the poles remained temporarily reversed, with a field strength that varied below 28% of today’s value, only to reverse back again over the course of about 250 years. Because the time flip was centered on 42 kiloyears, the researchers decided to name this the Adams Transitional Geomagnetic Event, after author Douglas Adams.
There has been much speculation that the weakened magnetic field during the Adams Event may be associated with changes in climate, faunal extinctions (including that of Neanderthals), and the migration of humans from Central Asia to Europe. Australia saw a major extinction of its megafauna that peaked at roughly 42,000 years ago, suggestive of a link to the altered rainfall that the Adams Event seems to have triggered in the Southern Hemisphere.
Current Events and Future Portents
The geomagnetic field has lost 30% of its intensity over the last 3,000 years, and is predicted to drop to near zero over the next several centuries. Already we can see the effects of a weakened magnetic field on satellites in orbit. In the Atlantic Ocean between South America and Africa, there is a vast region of Earth’s magnetic field called the South Atlantic Anomaly (SAA, Figure 2) that is about three times weaker than the field strength at the poles. This is a region where satellites consistently experience electronic failures. The SAA was first noticed in the 1950s, and since then it has decreased in strength by a further 6% and has moved to the west. Scientists are investigating what is going on inside Earth’s core that might be causing the SAA. One possibility is that there could be a vast convection in the southern portion of Earth’s liquid metal outer core that may be pushing the magnetic field out from the South Atlantic region. Another possibility is that a mini-polarity reversal is taking place in this region.
The SAA may be portending what is to come in the near future. If trends continue, the planet’s magnetic field could reverse again in one or two millennia. Even if it does not (Brown et al., 2018), the field may continue to weaken, and within a century we could be faced with serious problems. The decrease in geomagnetic field strength is much more important and dramatic than the reversal, and it is critical to understand whether the present field will decay to zero in the next century because, if it does, we will need to prepare.
For the first time, astronomers have detected a powerful, 1,000 km-wide hurricane of plasma (electrically charged gas) in Earth’s upper atmosphere (Zhang et al., 2021). The “space hurricane” raged for nearly 8 hours on Aug. 20, 2014, swirling hundreds of miles above Earth’s magnetic North Pole (Figure 4). Formed by the interaction of magnetic field lines and the solar wind, the hurricane was invisible to the naked eye, but four weather satellites that passed over the North Pole detected a formation analogous to a terrestrial hurricane. The space hurricane was shaped like a funnel with a quiet “eye” at the center, surrounded by several counterclockwise-spinning spiral arms of plasma. The space hurricane rained electrons directly into Earth’s upper atmosphere.
Until now, it was uncertain that space plasma hurricanes existed. Tropical storms are associated with huge amounts of energy, and this space hurricane was evidently created by unusually large and rapid transfer of solar wind energy and charged particles into the Earth’s upper atmosphere. While this is the first observed space hurricane, they may be common events. They have the potential to impact known space weather effects, such as increasing drag on satellites or disrupting GPS and radio communications systems. By altering the orbits of “space junk” objects, they pose a potentially serious hazard to orbiting space stations.
Brown, Maxwell, Monika Korte, Richard Holme, Ingo Wardinski, and Sydney Gunnarson (2018). Earth’s magnetic field is probably not reversing. PNAS May 15, 2018 115 (20) 5111-5116; https://doi.org/10.1073/pnas.1722110115
Cooper, Alan et al. (2021). A global environmental crisis 42,000 years ago. Science 19 Feb 2021:
Vol. 371, Issue 6531, pp. 811-818 DOI: 10.1126/science.abb8677
Nowaczyk, Norbert et al. (2012): Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments. Earth and Planetary Science Letters, 351-352, 54-69, https://doi.org/10.1016/j.epsl.2012.06.050.
Olson, P., and H. Amit (2006). Changes in earth’s dipole. Naturwissenschaften 93, 519–542 (2006). https://doi.org/10.1007/s00114-006-0138-6
Zhang, QH., Zhang, YL., Wang, C. et al. A space hurricane over the Earth’s polar ionosphere. Nat Commun 12, 1207 (2021). https://doi.org/10.1038/s41467-021-21459-y