Glacier, Laban Centre, London<br />Sylvia, Royal Opera House, London

Global warming is a big subject. Sadly, writhing around the floor in engine oil, while briefly entertaining, is unlikely to help

You wait half an hour for the most arresting image in Maresa von Stockert's latest show, Glacier, and it's almost worth the wait: a pouting, Beyoncé-like strut for seven dancers in pristine white clothes, wielding large, glossy black oil cans. It's the girls that draw the eye, each hugging and caressing her can as if it were an £800 handbag and squeezing it lasciviously between bare thighs. At an invisible signal, each removes the cap and does a butt-wiggling walk, shouldering the can like a gun as thick dark gunge dribbles down her back and legs. Soon the floor is so slimy it's impossible to stand. Limbs flail and slide, bodies collapse and collide. Ah, yes, the petroleum-reliant world is losing its grip. We get the message.

There's a good reason why dance makers rarely engage with political issues: movement, even with words to help, is just too blunt an instrument. Some years back, Darshan Singh Bhuller had great success with a piece about the Bosnian war, but he cleverly hitched a ride on the Romeo and Juliet story. Global warming is a bigger, more nebulous topic. Yes, we should be worried about it. But all dance can do, on this evidence, is wring its hands and fret.

There are other images that stick – so to speak – and maybe that's a small triumph in itself. In a sequence about throwaway plastic, a dancer secretes empty drinks bottles in the sleeves of his jumper. Wrapping his double-length arms in knots, his solo not only evokes the struggles of a tar-damaged seabird, but a scuzzy Oxfam version of Martha Graham's famous jersey-dance in Lamentation.

Earlier, to the baroque strains of a Purcell aria about "everlasting snow", performers swivel slowly on shards of polystyrene on a shiny black floor, as if adrift on diminishing ice floes. Later, though, they're using them as Frisbees, and then surfboards, which is fast and a lot of fun, but seems irrelevant. The choreography, when it gets stuck into something more dance-like, is often ungainly, though there's a strong duet for two guys balancing on a too-small triangle of "ice".

Stockert and her designer Naomi Wilkinson have evidently looked hard at the possibilities of polystyrene sheeting – a substance which, we are reminded solemnly in the opening spiel, is "a material derived from petroleum, far more durable than us". Adrian Plaut's seductive lighting does indeed transform this cheap packaging into suggestions of a serene arctic world, a world that, under threat, is suddenly so precious.

Stockert has used text in previous work, and doesn't stint on it here. Though it's admirable of performers to be able to deliver lines while dancing, their strenuous efforts not to breathe too heavily into the radio mic often distract from what they are saying. The hard fact is that however slyly and sardonically you spin your ideas, the words "environmental change" make a plonking verbal trope.

A less pretentious dose of nonsense is offered by the Royal Ballet. Sylvia, the 1952 ballet by Frederick Ashton, is looking even emptier and sillier than it did when rescued from oblivion three years ago, now that curiosity is sated.

The plot, based on classical myth, tracks the amorous adventures of gods and their acolytes: a lovelorn shepherd who spends most of his time lying on the floor with an arrow in his chest, and an Amazonian who, in a tutu fashioned like a pair of swag curtains, romps about the Roman countryside chasing and shooting dinner. Ashton even writes a duet for a pair of goats. Too camp to be taken seriously, not quite camp enough to be funny, Sylvia is, frankly, a bore, albeit one stuffed with intricate steps intended as a challenge to Margot Fonteyn.

The high reputation of Delibes' thin, over-pretty music, what's more, is simply baffling. "If I had known this music before," wrote Tchaikovsky, "I would not have written Swan Lake." Thank goodness, then, that he didn't hear it till Swan Lake was in the bag.

'Glacier': touring until 11 April. Tour details: 'Sylvia': ROH (020-7304 4000) in rep until 31 March