(posted in Plants & Fungi)

Wow, that's given me some crazy mental images! I have no idea, and it's probably something best known to horticulturalists. As a plant ecologist I can tell you that wild daffodils are much smaller than the cultivated form that you find in gardens, though I would argue that they're also much prettier. I tend to find the big, showy ones a bit gaudy in comparison. Gardeners over many centuries have selected the largest daffodils to breed, and gradually created strains with ever larger flowers. How large any single flower gets though will be down to a mixture of the genes for size that have been selected for, and the particular environment it's grown in. I presume there's some sort of limit, at least until a new mutant giant daffodil emerges!

That's a tough one! There are more than 200 species of grass in the British Isles; almost all of them have English names, though these have been largely forgotten as people in the modern world are less concerned with their relative value for grazing livestock or making hay. A wealth of knowledge about grasses has been lost in the last century.

I would still guess at a species of grass, perhaps Holcus lanatus, which is commonly known as Yorkshire fog, though it occurs throughout the UK. It's softly hairy and easy to identify among other grasses. That said, I don't have a definitive answer, and others may have different opinions.

That looks like some kind of fungal infection in the leaves, but it may be nothing serious, perhaps only the tree equivalent of a nasty cold. There are some aggressive diseases affecting European oaks, but the main symptom to look out for is bleeding sap from the trunk, either from a crack or seeping out and forming a tarry spot.

(posted in Plants & Fungi)

It sounds like you've made some great observations, and discovered something really interesting!

First of all, I have to admit that I don't know the answer... but it could be related to the decomposition of the insects. When an insect gets trapped, the plant needs to digest it (through excreted enzymes) and absorb any nutrients which come out (through pumps in the cell walls). I would guess that these proteins are directed only to where they are needed, which would save the sundew having to make them unnecessarily. What you might be observing are vesicles containing bundles of these chemicals, which the plant can then move around quicky whenever a food source becomes available.

Mosses are essentially the same as other plants in terms of their requirements: light and carbon dioxide for photosynthesis, water, minerals from soil and oxygen for respiration. Where they differ is in their ability to grow with a tiny amount of soil, often trapping it themselves, and are able to survive in deep shade.

I'm not a marine biologist, but I would guess the sudden depth change has a major part to play in this. When deep sea fish are brought to the surface they inevitably die - they effectively explode because their cells are adapted to conditions of high water pressure, so all their tissues turn to mush. For many of these very deep-dwelling fish species we didn't properly know what they looked like until exploratory vessels were sent down to film them in their natural habitat.

I can reassure you that they're not bed-bugs, which are actually several millimetres in length and quite distinctive in appearance.

Dear Emma,

You're quite right that mammal behaviour is a difficult field to break into, at least if you're aiming for the academic route. This is for a very simple reason: there isn't much funding. The scientific community in any particular field can only be maintained by the money that agencies put into it, and without the commercial resources available in some areas (e.g. medicine and genetics) or a pressing demand for new research (e.g. climate change or biofuels) the number of careers that can be made is limited. Add to this the fact that mammal behaviour is one of the subjects that most excites people and attracts lots of applications, and you end up with a very tight squeeze on resources.

I don't say this to discourage you, only so as you appreciate that you might need to prepare yourself to consider alternative options. While not your first love, would you be willing to consider a career in animal welfare, wildlife management or something similar? Are there other areas in the biological sciences that interest you? If you can see other avenues that you would be happy to follow, then you should go for it, knowing that you have a reasonable fall-back if required. It is also true that having done an MRes (or even a PhD), a large proportion of people decide that the academic life isn't for them, and move on elsewhere. This is true throughout academia, so I always advise my students not to invest all their hopes in a single career path.

In terms of getting on a PhD programme, just having an MRes is an immediate advantage, especially if it leads to some scientific publications, though that's seldom possible within a year. Most important of all will be the academic references you receive at the end of it. Being in a good research group is therefore especially helpful if a well-known scientist can recommend you for a PhD. What you actually do isn't as important as the general skills you gain from doing independent research; when you start a PhD it's likely that you'll have to pick up new techniques from scratch anyway.

Good luck - you clearly have the enthusiasm and the commitment, so I hope it works out for you!

This sounds quite normal to me - they're probably tree slugs, feeding on the algae on the tree bark. Try going out at night and see if they're still there. Most tree slugs climb upwards at night to feed, and they're probably huddled together in the daytime for mutual protection and to stay damp (so not really 'nests').

A point worth making is that evolution is the best mechanism known for selecting appropriate traits for survival. Rather than taking the countless species of coral and trying to breed new varieties of each, this process is going on right now on every coral reef. If evolution can't keep pace with environmental change then it's unlikely that a laboratory selection programme, which necessarily requires smaller populations with a more limited gene pool than in the wild, will succeed.

It sounds like a zoology degree would be most suitable for you. Modern zoology courses include a large amount of genetics and conservation, whereas a genetics degree probably won't teach you much about zoology. Have a look at the undergraduate programmes at some universities and I'm sure you'll be able to find a course that suits your interests.

I've never heard anything about blue clothes before - it sounds implausible to me! As for machinery, I imagine they would investigate anything noisy in the vicinity of the hive to make sure it wasn't a threat.

You're right, there are solitary ant species, in the same way that there are solitary bees and wasps. The only ones I've encountered are desert ants in North Africa, but I'm sure there must be others.

(posted in Plants & Fungi)

Please let us know where you took the photo.

The first question is where you live now, and then where you lived as a child. Lobelia inflata is native to the eastern USA so we don't ever see it in here in Europe (plus it's a restricted species). What you describe sounds like Caryophyllaceae (one of the campions), so check in your flora to make sure it isn't a Silene species or something similar.

Incidentally, I assume you know that Lobelia inflata has many interesting properties, including that it's good to smoke, hence it being restricted over here. It's also an emetic, meaning that ingesting even small amounts makes you vomit. Make sure you can tell the difference before attempting either!

The key word is 'maybe' - there's no evidence that life ever existed on Venus or any other planet. Some planets went through phases similar to the chemical and climatic conditions on ours, but life is still fundamentally improbable. Also, what's to say that life on another planet would require the same conditions as our own?


You're right to be worried, and realistically the answer is a bit of both. There are some changes we have made to the world that are effectively irreversible - geological timescales would be required to return them to their former states. But we still need to take action now. The world of the future will be greatly changed, but what we do in the coming years will determine what we end up with.

As Paolo rightly points out, the world has changed a lot during its history, such that it's impossible to call any state 'normal' or 'natural'. Ice ages, continental movements and volcanism have all radically altered the the climate and distribution of life before. Around 98% of species have gone extinct - almost all of them due to 'natural' causes (i.e. long before humans started affecting the world), yet more have evolved to take their place. Life will persist - the real question is whether our species will still be around to see it.

I wouldn't worry about it, you won't have poisoned them and they'll be perfectly safe to eat. Gardening advice should always be taken with a pinch of salt; a lot of the time it's just about habit and personal preference.

The shape of the flower itself has little to do with the resulting fruit. What matters is the number of ovules in the ovary. In many flowers you can work this out by cutting through the ovary and looking at the number of cavities or partitions with a hand lens. With the Rutaceae (the family which includes citrus fruits) you might need a microscope, but you can count the number of ovules and divide by two to work out how many segments there will be.

This should probably be moved to a different forum - placozoans are primitive Metazoans and slime moulds aren't fungi (I think they're Archaea).

(posted in Plants & Fungi)

The term 'essential oil' is not a botanical one, but refers to any aromatic extract from a plant, and is meant to refer to the 'essence' of a particular plant. Oils can be produced by a wide variety of plant parts and organs and serve a broad array of functions.

(posted in Plants & Fungi)

Good question. The spines are not hard when they first appear, and only toughen up when they have fully developed. They are therefore probably bent away from the other parts of the plant before becoming hard. This is easier to understand if you know that the spines are actually modified leaves (the 'leaves' of a cactus are actually stems) and therefore they can change during development.

Yep, I'm stuck for anything that would fulfil all four criteria. Insectivorous plants aren't usually invasive because they're usually adapted for very low-nutrient environments and don't have the powers of rapid growth and reproduction that usually favour invasive species.

You're completely correct in saying that 'global warming' doesn't affect the whole globe equally. While some parts of the world have been warming at an incredible rate (such as Siberia or Antarctica), others have seen hardly any change in the last fifty years, or have even cooled slightly. One of the unfortunate quirks is that most of the eastern United States, including New York and Washington, lies in an area where the climate has barely changed at all. I can't help but believe that the political inertia in the US is related to the lack of obvious changes on the doorstep of the main decision-makers. In Ethiopia or Sudan it's not merely a theoretical process - it's the cause of massive displacements of peoples and persistent crop failures.

(posted in Plants & Fungi)

A quick couple of replies:

1. I've never heard it said that the central tissues are capable of doing anything useful, but if you can find where you saw this then I'd be keen to see. I'm sceptical as all the cells are killed off and the membranes which control the processes will have entirely withered. It's also not advantageous to the tree to have the heart full of water as it will be heavy and encourage the spread of pests or fungal diseases.

2. There's definitely an active aspect as they go to such lengths to make sure it's packed with noxious chemicals. Once it's there, the maintenance costs are nil. I can't see an easy way to test the resource-hoarding theory, but it's certainly plausible. If they wanted to get rid of it, all they would need to do is let the fungi in!

(posted in Plants & Fungi)

An update - I've since learnt that domesticated figs don't require pollination, they produce their fruit clonally. Sorry for the change in answer, but I'm glad to be able to provide a correct one now!

(posted in Plants & Fungi)


You're right in thinking that the heart of most trees is dead. There are exceptions among other plants that produce trunks - giant bamboos, rattans, palms and tree-ferns have living tissue spread throughout the stem. But for normal trees, the internal part of the trunk is comprised entirely of dead cells. These are often packed with noxious chemicals to prevent fungi spreading through the tree, although this is not always successful.

Why do trees maintain such a large amount of dead material, often called necromass by ecologists? Strength and support are only part of the story, since hollowed out trees are often perfectly capable of persisting. One possible explanation lies in the fact that trees are essentially designed to be the great competitors of the plant world. They grow taller than anything else, shading out other plants. Their root systems not only go deep but also spread wide and pilfer nutrients from the topmost litter layer. The dead trunk is yet another manifestation of this - they hoard nutrients in this, locking them away and reducing the fertility of the soil so that no competitors can survive. They don't need it - but they're not going to let any other plants get at it either!

I've now consulted one of Britain's leading slug watchers, Chris du Feu, who sent the following fascinating information about these noble creatures:

From the description of the behaviour of the slug it seems likely to be a yellow slug (Limax flavus). This species feeds on fungus, lichen, mould, algae etc. and is often found in houses, typically in rooms where there is some dampness (such as under the sink in the kitchen). Although they are very widespread and very often found living in or around human habitations, they are grossly under-recorded.  This may be because they are so highly nocturnal. It is rare to see one active at any time from just before first light until after dusk. They are reputed to have a strong homing instinct although this reputation seems to be anecdotal rather than well established. Slugs of this species have markings which are individually identifiable and this will allow you to check whether your slug is the same individual returning to its base or whether it is just a good place for slugs to be and when one slug leaves another takes its place. Take a digital photograph of the slug and evict it. The next night take another photograph and see if it is the same individual.

Once you have established whether it is the same individual, you might ask what mechanism it uses to find its way home. It could be the slime trail. By removing all traces of this trail you could see whether it reduces the slugs ability to find its return route. Alternatively you could take the slug a little further away each night to try and discover the distance from which it can find its path home. Because of the individual markings the possibilities for studying this species are very great indeed.

Incidentally, this species of slug is far from being a pest. It avoids green plants in favour of other matter. I have read of an Australian who has a jar in his shower room where some of these slugs live. At might they emerge to remove all the mould that grows on the grouting between the tiles. They return to the jar before first light leaving the grout clear of mould and glistening with the dried slime trail. A labour saving, environmentally cheap solution to the tile cleaning problem.

Do not underestimate the humble slug! Many species of slugs make nocturnal journeys to their favourite foraging sites, returning at dawn to hide in leaf litter or damp crevices. Some woodland slugs climb to the tops of trees each night, where the algae on tree bark are more abundant and diverse. In suburban areas it's common for them to graze on the algae of rooftops, and if you look at the right time then you might be able to find the route that they use to climb your house, since they tend to follow each other along a common trail (it saves on slime).

The slug you keep finding is either on its way to a better feeding place, and you happen to keep disturbing it, or else your skirting board is a lush algal farm.

(posted in Plants & Fungi)

Well, I can certainly tell you that commercially produced figs contain seeds, and I would expect that they are pollinated. It's actually not difficult to keep populations of fig wasps going in fig plantations because they tend to produce fruit all year round so there is always somewhere for the wasps to breed.

I'll second Paolo's answer. The reason trees are said to provide a CO2 sink is because they trap so much carbon in their trunks. This is dead tissue, but it locks carbon away. Any herbaceous plants will eventually die, so any small amount of carbon that was stored in their tissues gets quickly released back into the atmosphere.

Watching David Attenborough's series Life in Cold Blood, I was impressed and puzzled by the video of the jumping snake. I may have missed it, but I don't think he explained why the snake jumps.  Can anyone provide an explanation? Is it a mating or territorial display, or for prey capture, or some other reason?

(posted in General Biology)

It's also well worth bearing in mind that our agricultural strains of bananas have been in cultivation for thousands of years and have changed substantially from their ancestors. The way animals handle our crops may be very different from 'natural' bananas in the wild.

I seem to recall hearing that there was an Indian endemic fauna that was largely replaced when the subcontinent reconnected with the main land masses, leaving only a few regional endemics such as the garial. Certainly the flora of India reflects this - the south of the subcontinent contains a large number of endemics (especially the Western Ghats), but most other species are of Arabian or Malayan origin. I'm going to have to check the details but I'll fill in more later.

In short, India probably would be weirder if it had never reconnected with the main plates. Large islands such as the ones you list are odd precisely because they remained separate. South America is the exception that proves the rule - a very large proportion of the endemic fauna of the continent went extinct when it connected with North America.

(posted in Plants & Fungi)

This is a tricky one to answer. The species of fungi present in different parts of the world are quite variable, so you would have more chance if you asked a local specialist. I asked around the mycologists in my department, and received the following response:

Sorry, does not ring any bells. If the pileus (cap) was spherical it may be that this was a young fruit body that had not yet opened up - some Basidiomycete form toadstools with balled heads which then open out flat to reveal the gills in the mature form. Identification at that stage is a lot more straightforward.

This is a complicated set of questions, so apologies for only responding briefly to each. It may be better to split it into a few separate threads. Most of all, however, this can only be speculative because it’s impossible to do an experiment to test any of these ideas!

1. Would a 5 times faster growth of plants also result in 5 times greater density of herbivores and their predators?

Not necessarily, and in fact it’s difficult to predict what might happen due to the interactive effects on diversity and habitat structure. You would probably get more, but not a five-fold increase. Consider that on land, most plant growth is not eaten by herbivores, hence the world being largely green. Terrestrial plant growth already far outstrips the capacity of herbivores to eat it.

2. Would such a situation favour greater numbers of fewer species or similar numbers as in our world but of more species?

It is actually likely that the number of species would fall. This is a common finding in high-productivity environments. Compare, for example, a coral reef (resource poor) to a fast-growing kelp forest (resource rich) with a fraction of the number of species. I would expect a few plant species to dominate, leading to a corresponding loss of herbivore diversity. Again, however, it’s difficult to be certain.

3. Would faster plant growth result in a different atmosphere and different weather patterns?

Yes – probably a greater annual amplitude of variation in atmospheric C02. Weather patterns would be most influenced by transpiration (evaporation of water), which would increase, provided there’s somewhere for all this extra water to come from (which is a pretty big hole in the story).

4. Could 5 times greater numbers of larger predators result in other, larger predators that are likely to prey on say, tigers, eagle owls, bears or very large megafauna such as elephants?

No. All the evidence from modern nature and the fossil record suggests that food chains are constrained in length, and that higher productivity simply leads to a greater number of parallel chains rather than the development of super-predators at the top.

5. Would it result in humans cultivating much smaller tracts of lands and might the lack of neccesity to produce 'more food by having more hands' result in smaller family size and birth control because more children would actually be detrimental to the ability to feed the family on the existing land?
6. Would agriculture even be likely to be developed in a world so rich in natural resources?

No – more food would mean higher infant survival to adulthood and larger families. Otherwise the arrival of fertiliser, pesticides and herbicides in the third world would have caused a reduction in population growth, and that’s clearly not the case. Eventually, however, the human population would build up and stabilise regardless of the level of plant productivity. Similarly, there is nothing magical about plant productivity rates that necessarily led to the development of agriculture.

In the context of conservation, we often refer to flagship species rather than indicators - the species that the public can get excited about. They're also sometimes called 'umbrella species' because the implication is that by saving one you will probably conserve many more. There have been some successful cases, mainly using large and glamorous creatures (elephants, condors etc.) but not always (e.g. the Sinai Baton Blue butterfly in Egypt).

As Dave rightly observes, there isn't an indicator species that can be reliably linked to one specific community or ecosystem, and defining an ecosystem in the first place isn't as straightforward as it sounds. There is a great danger of being overly simplistic - is a rain forest without macaws less important than one containing them? - and there is also no guarantee that what works for one species will work for others.

As an example, take a favourite flagship species - the orang utan. These are commonly used to promote preservation of rain forests in Borneo and Sumatra. However, in Borneo, there is good evidence that some secondary forests (which have been logged) can support a greater density of orang utan as there is more fruit available. Saving the orang utan won't actually mean that the primary forests get saved at the same time. In my opinion it is far better to concentrate on special communities than single species.

Following on from Paolo's comments, it's interesting to note that other societies elsewhere in the world have very different systems. In Tibetan culture, it is considered normal for several husbands to share a single wife, while in many other regions polygamy (several wives for one husband) is preferred. Whether this is because of local resource conditions or simple cultural variation is a topic of great debate among anthropologists.

You are right to note that our closest ancestors are not monogamous, and even those related species that are (e.g. gorillas) have physical differences that suggest we are naturally different. That's not to say, however, that our species might not be changing - perhaps we are evolving to be monogamous?

They are fascinating organisms, aren't they? When you're dealing with something as simple as a flatworm, whether they 'feel' pain is debatable because it's such a subjective experience, and relies upon more than the presence of nervous tissue. Do they know when they've been cut in two? Probably. Do they suffer as a result? Certainly not in the same way as a human or any other organism with a more developed nervous system would.

Another good question - you certainly think of interesting things! You're right, plant leaves are highly variable, both within and between individual plants. This is one of the reasons why leaves aren't very good characteristics for identifying most plant species.

The development of a leaf isn't the same as, for example, the human hand. The plant has a lot of flexibility over the final appearance of the leaf, and can adjust it to make sure each individual leaf is tailored to the particular conditions it's found in. The overall layout of the leaf is genetically determined - the tissue types and pattern of veins - but after that it's up to the plant.

Most of the differences arise during the leaf expansion phase. As a leaf develops, all the cells divide and specialise into their particular layers very early on, often before you even recognise it as a new leaf. They do this while still incredibly small. Most of the 'growth' of the leaf from that point on is just expansion of existing cells, like inflating a balloon, but with fine control over how large each part gets.

For example, plants growing in shaded areas might adapt their leaves to be thin and as broad as possible to catch all the light they can, while those in direct sun can afford to be smaller and thicker, and therefore less fragile. They might adjust their defences - have you noticed that holly leaves are only spiny at the bottom of the bush, but smooth at the top? This is because they don't bother extending the spines when no herbivore will ever reach them.

You should go out and have a look at plants of the same species growing in different conditions, and you'll see what I mean. Look at the differences between the leaves, and I'm sure you'll be able to come up with reasons why the plant has chosen to adapt its leaves one way or another in any particular place.

(posted in General Biology)

Most of the mucus we produce in our respiratory tract ends up in our stomachs anyway, either coming upwards from our lungs or draining down from our nasal passages. Snot is only dried mucus, so it's essentially the same, although there might be more coarse particles stuck in it.

I suspect it's a common habit even among adults. Any glance at other drivers in a traffic queue will confirm this. I wonder why it's so common - is it, as you suggest, a means of saving on protein? Why has it become socially unacceptable?

I shouldn't be thinking about this so much but it is Friday!

(posted in Research and Careers)

I'm a university lecturer, and in an average week I'd work about 45-50 hours, but there are occasional weeks with much more, and very few when I work less.

(posted in Plants & Fungi)

Any green parts of plants are involved in photosynthesis. Usually this means the leaves, although in some plants the twigs and stems are able to photosynthesise as well. However, they use the CO2 to create sugars that can be transported anywhere in the plant. This occurs through the phloem, which is the equivalent of the bloodstream in plants, but obviously works very differently (they don't have a a heart!).

You're also correct that different plants can take up CO2 at different rates. In some cases this is because of the type of chemical pathway they use - trees, grain crops and cacti all make sugars through photosynthesis in slightly different ways. Grain crops like maize are the most efficient at taking up carbon for a given area of leaf. However, even with the same biochemical pathways, different plant species can be faster or slower.

Why aren't all species fast, you might wonder. Well, for the same reason that not all humans can run like Asafa Powell. Some of us can run quickly over short distances, some can run marathons, others (like me) are best just walking. Plants are much the same - they can't be good at everything. Cacti have slow photosynthesis because they're adapted for survival in the desert instead.

Many features of organisms don't need a selective reason to explain them, and I think this is probably such a case. We have five digits on each limb because we evolved from ancestors with five digits on each limb, and back in the distant past when the first mammals arose, there probably wasn't a powerful reason why five was much better or worse than four or six. The fact that it has remained unchanged suggests that there can't be a strong selective advantage to having more or less.

Deciduous trees tend to drop their leaves at around the same time each year - I believe that this is a photoperiodic response and not likely to be affected by climate change. I was out in a forest reserve in Nottinghamshire yesterday and the leaves are just beginning to turn here too.

As for absorbing more carbon, you have to ask where the carbon actually goes. The only way for trees to lock up carbon is for it to go into woody tissues - trunks and branches - and stay there. In fact, only a fraction of the annual photosynthetic production does this. Any carbon that goes into leaves, flowers or fruits will only decay and return to the atmosphere at the end of the year. This causes an annual carbon cycle, with the global atmospheric concentrations rising and falling, driven by the larger vegetated land masses in the northern hemisphere.

There is some circumstantial evidence that carbon may be 'fertilising' extra woody growth in forests, but more recent evidence has shown that tree growth rates in tropical forests are in fact declining. Other studies suggest that the rate of turnover in forests, that is, the cycle of tree death and regrowth, is increasing. This would mean that even carbon stored in wood will fall and be released more quickly. There is a lot of controversy - as a generalisation, atmospheric scientists tend to believe that trees are taking up more carbon, while most foresters disagree. The likely reason for the confusion is that any forest will reach a maximum carbon storage potential when it can hold no more. Even if some forests have taken up more carbon, they will not do so indefinitely.

On balance, I think that any extra carbon uptake by early leafing trees is likely to be of negligible importance, and will not mitigate against climate change.

(posted in Evolution)

There are a number of myths on this subject that I've heard from Creationists, and I can assure you wholeheartedly that this one is untrue. In fact, later in life, Wallace became an even more vocal advocate of evolution by natural selection than Darwin himself. If anything, Darwin's later writings became more conciliatory, for example by accepting the possibility that Lamarckian inheritance might occur as well. Wallace also went further than Darwin into the controversial territory of human evolution, even if some of his observations were spoilt by an obsession with the supernatural.

You can only tell from the flowers, so unless they come out it's a mystery, even to geneticists (plants typically don't have X and Y chromosomes in the same way as animals). Many (if not most) plants have both male and female parts, usually in the same flowers. These are referred to as dioecious plants.

Once you have flowers, the parts present in the middle tell you everything you need to know. The anthers (long stalks covered with dusty pollen) indicate that the flower is male, the central spike (there may be several) is the tip of the female parts. If there's only one or the other then it's a good sign that you're looking at a male or female flower. Be careful though - some plants, especially trees, have separate male and female flowers on the same plant, and in a few cases even have them on different branches. Make sure you look at more than one flower. Also, some plants bring out one sex before the other (usually the male flowers followed by the female flowers) so it becomes quite complicated!

Which particular plant were you thinking of? If it's something specific then I could give clearer advice.

(posted in Plants & Fungi)

This one is actually a bit easier... it depends on how many of the ovules get fertilised, i.e. how much pollen the flower receives. It's not possible to say how many seeds will be in the average fruit.

(posted in Plants & Fungi)

A bit of a specific question this, and I imagine that it will vary quite a lot between different flowers. In order to get an estimate, I would wash an anther in a precisely-measured amount of viscous fluid (thicker than water - it has to hold the grains in suspension), then take a small amount and dry it on a microscope slide. After counting the number of pollen grains in a known amount of fluid you could scale this up to the whole anther.

Some climate scientists believe that we should actually be in a period of global cooling at the moment, if the usual cycles in temperature were still being followed. The fact that the earth is warming up, and at such a dramatic rate, is therefore more likely to be due to human effects than to a natural process. As Graeme says, we can never be 100% certain because the climate is so complex, but all the best evidence at the moment suggests that it's getting hotter and we're to blame.