As we are all aware, our climate is rapidly changing and these changes have come about through humans lifestyles. Scientists, economists and politicians worldwide are investing millions of dollars into prediction techniques, can we estimate what changes will occur and what we can do to mitigate them? One prediction method is to look into our planets history. Planet Earth has experienced numerous cycles of cooling and warming over the past millennia. Looking at historic climate and the effects these climates had on the world’s fauna and flora at that time, can we start to predict what might happen to our planets species (humans included) under today’s current climate change?
Planet Earth has been through constant glacial cycles, during the last million years for example there has been an ice age on average every 100,000 years. The last ice age ran from about 75,000 to 15,000 years ago. There has been times in our earth’s history where the climate has been cooler than today but also much hotter. On average warmer periods are associated with higher species radiation, but as the climate shifts from one state to another there has always been a large turnover of species diversity and numbers.
Furthermore, we understand the causes to our current rapid climate change. We have been pumping atmospheric greenhouse gases into our atmosphere through our livestock, transportation and industrial behaviours. These gasses disrupt the heat transfer between the sun and earth’s surface, disrupting the earth’s current climatic and weather cycles. However, as mentioned the earth’s climate has been changing throughout history, long before humans started to evolve and conquer. So what caused these historic climate changes?
Our planets cycle between glacial and interglacial periods can be explained through the Milankovitch theory. The Earth orbits the sun on long term cycles eg 100,000 years. The degree of how elliptical the orbit is changes, changing the amount of solar radiation the earth receives. The angle of tilt towards the sun also changes on a 41,000 year cycle. When the angle towards the sun is greater, seasons are more marked, summers get more energy and the winter less. The cycles are determined by the gravitational pulls from larger planets as our own planet circulates the sun. These cycles have set the base for what climates we can predict throughout the earth’s history, but there are other effects on our earth’s surface that can change the strength of these processes.
The geography of our planet has played a huge role in the changing climates the earth has witnessed throughout history. Firstly, 40 million years ago Antarctica was still connected to South America. Then 34 MYA Antarctica split and Drakes passage opens, setting up a cold current around Antarctica, isolating the cold water from the rest of the ocean currents. As the tectonic plate drifted south it gained a lot of snow and ice, reflecting a lot of heat back into the atmosphere therefore cooling the earth slightly, this is called the Albedo effect. Secondly, the rise of the Himalayas and Tibet have also had a huge role to play in our planets current climatic conditions. Around 45 MYA these areas were elevated, reducing connectivity between warm equatorial air masses and more northern temperate air masses, leading to less dissipation of heat, and so greater cooling in the more northern latitudes.
These geographical changes to our planet’s surface actually set the stage for more ice ages, intensifying the effects of the Milankovitvh cycles. This was true, in the earths not so distant past we have experienced many ‘ice ages’. However, now we humans are causing our planet to heat up (on average) and cause great changes to our earth’s climate cycles. We can look at what happened to our planets make up as we left the last glacial period and potentially predict what might happen as our planet enters a warming period currently.
The last ice age to encompass our planet was the Pleistocene around 18,000 years ago. Much of the Northern hemisphere was covered in large glacial and sea ice areas. It was not just the Northern Hemisphere that was effected, in a cold phase most of the water is locked up as ice and therefore there is a drier climate, driving the retraction of forests across the globe. This pattern of climate has shaped the species we have today. For example, lower latitudes accumulated taxa, acting as ‘museums’ of biodiversity with their more stable climatic conditions. In contrast higher latitudes have had a higher turnover of species because of the climatic shifts. Climatic variation across the globe drove speciation and gave rise to many new creatures.
From this information it is clear to see that changing climates lead to a change in species diversity and numbers. Different geographical areas and habitats will be affected differently by the changing climate. For example lentic (lake) and lotic (flowing) habitats impose different constraints on how invertebrate populations can adapt to changes in the climate. Species from lotic environments are better able to disperse and adapt to changes whereas in the more stable lotic habitats species have lower dispersal abilities. Species life histories and traits will affect their ability to adapt to changing climates and will predict whether or not the species will be able to persist.
As mentioned earths biological history has been in constant flux with changing climate and therefore drastic changes with the planets fauna and flora. These changing climates have been used as explanations for many of our planets mass extinction events. For example, the asteroid that killed the dinosaurs did so as clouds of particles got into the atmosphere and drastically changed the planets climate wiping out entire families of species. However, many anthropologists and historical scientists are now looking into the links between both climate and expanding human populations and the role this had on the planets mass extinctions. Was it climate change that caused species to die, or was it expanding/developing humans?
Many species became extinct around 4000 years ago, just as the planet started to warm. But this is also the time humans started to radiate across the globe. Many of the species that died had been on earth for a long time and were able to survive many fluctuating climates through different glacial and inter-glacial periods, yet they disappeared coming out of the last ice age. The same fate was met by many of the world’s mega-fauna, the best example perhaps being the woolly mammoth.
Mammoths evolved around 200,000 years ago and made their last stand around 4000 years ago on Wrangle Island in east Siberia. As the earth warmed coming out of the Pleistocene, mammoth populations contracted to more suitable northern habitats. However, their final extinction was not preceded by further reductions in genetic diversity and effective population size suggesting the final knock off was extremely rapid (not to be expected from climate change) and therefore we can suggest that this final push actually came from human hunting pressure.
Further evidence for the causes of mass extinctions comes from Australia. Even though Australia’s climate changed some 45,000 years ago, it wasn’t a very remarkable upheaval. It’s hard to see how the new climate alone could have caused such devastation to the nation’s mega-fauna. Australia was full of amazing ‘giants’ with a 200kg kangaroo, marsupial lions and tigers, a flightless bird twice the size of ostriches and the diprotodon, a two and a half ton wombat. These giants appeared in Australia more than 1.5 million years ago and since survived at least 10 ice ages. So why did all 90% of the countries megafauna disappear around 45,000 years ago?
The answer is humans. This is around the time ancient humans settled in Australia. Within a few thousand years of human presence, virtually all of these giant species vanished. Of the 24 Australian animal species weighing 50kg or more, 23 became extinct. This same pattern of human arrival and animal disappearance was seen across the world. When Maoris reached New Zealand 800 years ago, almost all of the local mega-fauna went extinct along with 60% of all bird species. In South America within 2000 years of sapiens arrival, most of the unique species were gone, losing 50/60 of their ‘giants’. More evidence of a human induced extinction is the fact that when climate change causes mass extinctions, sea creatures are usually hit as hard as land dwellers. Yet there is no evidence of any disappearances 45,000 years ago.
Moreover, all of this evidence suggests that although climate did for some species start restricting populations, humans finished them off through over harvesting and habitat destruction. Organisms were able to cope against thousands of years of climate change, however the added pressure from humans lead to organisms reaching a tipping point where they could not come back. Unfortunately, this is exactly the same as we are seeing in our current climate change situation.
Many critics say that our fauna and flora species will be able to cope with climate change and that we shouldn’t be worried. The fact is that climate change is just one stress factor our planets species are having to deal with. Species can adapt to climate change by finding new suitable habitat or even adapting their behaviour. However, in previous climate change these adaptations were allowed many years to materialise. The rate of change we see today is unprecedented. As well as the rate of change, our planets species are already under immense pressure from over-harvesting, habitat degradation and spread of diseases.
The lessons we can learn from historical climate change is that nature is resilient and can withstand the earth’s natural changes. However, we have seen before that these changes coupled with pressure from spreading humans causes mas extinction. We are currently in the midst of another global mass extinction, through extreme climate change and human pressures our world’s species are disappearing. In order to slow the destruction we must actively help species cope with the destruction. We can protect natural areas from human pressure, we can allow spaces for animals to move in reaction to climate change, we can apply pressure on human populations that are harvesting endangered species but much research needs to be done in order to set priority areas in order to direct funding.
All these things will take time, something that many species don’t have. As global citizens it is our duty to not only be concerned about the destruction we cause but do what we can to stop it. As individuals we must spread the word on these issues and learn how we can adapt our life’s in order to give these species a better chance of survival. We have seen what happened to the world’s historic species as humans became dominant, but yet we are repeating our mistake against the species we still have left. Let us not make the same mistakes this time.