Euroturtle

Are there signs, or environmental indicators, that are acting as a warning?

‘Global warming: early warning signs’

	The Himalaya The Khumbu Glacier (on a popular climbing route to the summit of Mount Everest) has retreated by over 5 km since 1953. In the central and eastern Himalaya, glaciers are contracting at an average rate of 15 m per year, and could be gone by 2035 if this trend continues – with serious implications for populations who depend on glacial meltwater for drinking supplies, etc. Meanwhile, glacial lakes are swelling in Bhutan, increasing the risk of catastrophic flooding downstream.
Alaska, USA Most of the state is underlain by permafrost (permanently frozen soil). Thawing permafrost is causing the ground to subside (by 4–10 m in some places), undermining buildings, roads and other infrastructure. In some coastal areas, wave action is undermining cliffs softened by permafrost melt, increasing the risk of flooding for native communities. In the interior, forests of spruce and birch are taking on a ‘drunken’ appearance on softening ground, and trees are dying as they succumb to waterlogged conditions. A ‘drunken’ forest on ground softened by melting permafrost, outside Fairbanks, Alaska.	Copyright © Ashley Cooper/ Alamy Images;
	Chokoria Sundarbans, Bangladesh Rising sea levels have flooded about 7500 hectares of coastal mangrove forest during the past three decades. Global sea-level rise is aggravated by substantial deltaic subsidence in the area due mainly to human activities, such as reduced sediment supply following dam construction upstream for irrigation schemes, and the over-extraction of groundwater.
United Kingdom The average flowering date of 385 British plant species has advanced by 41/2 days during the 1990s compared with the previous four decades; 16% of the species flowered 15 days earlier on average. Over a 20-year period (between 1968–72 and 1988–91), many bird species have extended the northern margins of their breeding ranges in the UK by an average of 19 km.
	Monteverde Cloud Forest, Costa Rica A reduction in dry-season mists due to warmer Pacific Ocean temperatures has been linked to the disappearance of 20 species of frogs and toads, upward shifts in the ranges of mountain birds, and declines in lizard populations.
Antarctic peninsula Adélie penguin populations have shrunk by 33% over the past 25 years in response to declines in their winter sea-ice habitat. Adélies depend on sea ice as a resting and feeding platform. They are being replaced by gentoo penguins (a sub-Antarctic species that has begun to migrate towards the pole) which thrive in open water.

The notion of a link between climatic conditions and the behaviour of plants and animals (e.g. the growth of trees or coral) and the composition of natural communities or ecosystems (the type of vegetation in a given area, say) is fundamental to the use of proxy data to reconstruct past climates. Some examples of biological responses to recent climate change were included in the box below. Here we should be wary of jumping to conclusions. Such changes involve complex living systems that can respond in complicated ways to a great variety of other pressures. Particular caution is necessary wherever records are of short duration, which in this context means less than a few decades.

Well aware of this striucture, and having conducted a literature survey of papers documenting biological and ecosystem changes on this sort of time-scale, the IPCC concluded (with high confidence) that the following observations are related to recent climate change:

• earlier flowering of plants, budding of trees, emergence of insects and egg-laying in birds and amphibians;

• lengthening of the growing season in mid- to high latitudes;

• shifts of plant and animal ranges to higher latitudes and higher altitudes;

• decline of some plant and animal populations.

Many biological phenomena (e.g. leaf bud burst and flowering in plants) cannot proceed until a minimum temperature has been reached over an adequate length of time. Changes in the timing of such events are easy to observe and monitor and can provide sensitive indicators of climate change. Studies from various regions and ecosystem types tell a consistent story. For example, from Scandinavia to the Mediterranean and across North America, the growing season for plants has increased by 1–4 weeks over the past 50 years; spring comes earlier, but leaf fall in deciduous plants is delayed. Many animal life cycles also depend on temperature; in the UK, for instance, it seems that aphids now appear on average a week earlier than 25 years ago.
Migrating animals, especially butterflies and birds, benefit from keeping pace with the changes by arriving earlier in their summer habitat, so that food such as pollen and insects is available at the right time. Many are responding in just such a manner. However, there are signs that, in some cases, important inter-dependencies may be slipping ‘out of sync’ as the species involved respond to changed conditions in different ways.