When the lands west of the city of London were still left to their own devices, they were crowded with ‘dense woods and forests’, according to a charter issued in 785 on behalf of the Anglo-Saxon King Offa of Mercia. To the west of this belt, which ran from the Fleet river to what is now Mayfair, the Ela burn, later known as the Tyburn, made its way south and then turned east below the woods, losing its thread in marshes before entering the Thames. In doing so it formed an island, upon which Edward the Confessor built Westminster Abbey. At spring tides the marshland was flooded.

Later in the Saxon period, fourteen ‘leprous maidens’ founded a hospital on the northern margin of the swamp. Pious as well as leprous, the women dedicated their hospital to St James the Less. It survived until Henry VIII deported the lepers to Suffolk, built St James’s Palace upon the site and drained its southern hinterland so as to create ‘a nursery for deer’. In Henry’s day this area was apparently an open field, with a central avenue forming the embryonic spine of a park.

Further steps in that direction were taken in the early seventeenth century by James I, who ordered a decorative arch to carry the Tyburn into a reach known as Rosamond’s Pond. This pool later gained a reputation as a trysting place for lovers, particularly doomed ones, which gave rise to its alternative name: Suicide Pond. The park quickly acquired a pastoral mythology too. During the reign of Charles I, a French courtier named de la Serre reported that the park ‘is full of wild animals, but as it is the place where the ladies of the Court usually take their walk, their kindness has made the animals so tame, that they all submit to the power of their charms rather than to the pursuit of the dogs’. The fauna of the park included the exotic inmates of a royal menagerie, among them a crocodile and an elephant.

Charles I passed through the park on his last procession, from St James’s Palace to the scaffold at the Palace of Whitehall. The republican Commonwealth government stripped it of some of its ornament, renaming it James Park and felling for fuel many of the trees that had grown up in it – de la Serre had remarked upon ‘the shade of an innumerable number of oaks’ – but re-stocking the deer. It was Charles II who finally made the park modern, turning it into a playground for Restoration fashion, frivolity, gossip and intrigue. The menagerie had gone, but there were aviaries instead, giving a name to Birdcage Walk and introducing the strain of exotic birdlife that flourishes in the park today. A Russian ambassador initiated pelican diplomacy, presenting a pair of the birds to the king and starting a tradition that foreign envoys still uphold. By this time Britain was into the ‘Little Ice Age’; the diarist Samuel Pepys recorded skaters in St James’s Park, the first he had seen in his life, on 1 December 1662.

Charles had the park geometrically remodelled on the lines favoured by French royalty, with the trees parading along avenues and the water tidied into a ruler-straight canal that bisected the park from east to west. In plan view, St James’s Park now resembled a kite. It is said, however, that even the landscape architect André le Nôtre, who laid out the gardens at Versailles and Fontainebleau, declined to impose a similar degree of order in this pastoral enclave, maintaining that ‘the natural simplicity of this Park, its rural and in some places wild character, had something more grand than he could impart to it’. The park continued to impress visitors with its rural charms, including milkmaids stationed at the Whitehall gate selling milk straight from the cow, twice daily, at noon and evening. A throwback to earlier sporting traditions occurred in 1739, when an otter, said to be five feet long, was hunted with hounds and speared to death; but the harmonious relationship with wild animals developed by the ladies of the first Charles’s court appears to have endured, for a foreign visitors’ guide of the period advised that stags and deer would feed from the hand.

In 1771 a French observer noted that on the southern side of the park, ‘nature appears in all its rustic simplicity: it is a meadow regularly intersected and watered by canals, and with willows and poplars without any regards to order’. This insistent theme of natural simplicity was taken up in the late 1820s by John Nash and incorporated into the grand design that linked St James’s Park to Marylebone Park, refashioned and renamed Regent’s Park, via the new Regent Street. The canal was softened and widened into a lake with a plausibly irregular shape. Avenues were replaced by winding paths and flowerbeds by shrubbery. The Ornithological Society of London contributed birds and a cottage for their keeper; both the building and the post remain in existence. As a landscape the park is much as Nash left it, and current management aspires to be faithful to his spirit.

Many of the 1,700 trees in St James’s Park and its neighbour Green Park are planes: some date back to Nash’s day, and could possibly last the course of this century too. Within the parks the trees are generally in good condition, but on the edges they are not. Along Pall Mall, the ceremonial boulevard that runs westwards from Buckingham Palace along the northern edge of St James’s Park, the trees suffer from poor soil, vehicle exhaust pollution and salt spray from the road in winter.

For the public, the park remains the amenity it has been since Charles II first allowed them in. It offers sun, shade, space and an intimation of nature. The animals that feed from the hand are grey squirrels rather than fallow deer, but the sensation for the feeder must be much the same. Many of the birds also approach their benefactors with a swagger, and some may even perch as well. It may not amount to a vision of the harmony between man and beast before the Fall, but it does offer a welcome suggestion that even in the heart of a huge city, interactions usually dominated by anxiety and suspicion may turn out better than expected.

The value of the park to people in central London will grow with the heat. By the 2080s average summer temperatures in the Greater London area are projected to rise by up to 5.5°C, an increase of more than a third over the present average of just under 16°C, according to a study by the UCL Environment Institute. With daily highs pushing thirty degrees, London summers will be as hot as those of Naples or New York today. Winters will be nearly 3.5°C milder. Rainfall will be reduced in summer to half the levels of a hundred years before, but will increase in winter by over a fifth; over the year as a whole, rainfall will decline (by over twelve per cent from its current levels of around 700 millimetres) and snow will be seen only in Dickensian Christmas images – which might maintain their nostalgic appeal. Under these pressures the soil in London’s parks and back gardens would lose a quarter of its moisture, measured over the year, and nearly forty-five per cent in summer.

By the later stages of the twenty-first century London could have a climate similar to that of Marseille in the twentieth century, and Mediterranean leanings in its character too. As the city heats up, people in central London will seek shade rather than opportunities to sunbathe. Indoors they will be comfortable, because of the measures that will be taken to adapt buildings to the heat, but they will still want open air. For visitors, the parks will be oases; and St James’s Park will be the oasis at the capital’s heart, between the Palaces, Parliament and the centre of the city.

The park’s daily rhythms will alter. Already there are signs that in hot summers, visitors are starting to prefer the evenings. The fashionable parades of the Restoration park might in time reappear as an evening paseo like those performed by smartly dressed families and keen youth in the towns of southern Europe. Opening hours, entertainments and security arrangements could adjust over time to the new routine.

Underlying the new social opportunities offered by warm summer evenings, however, is a phenomenon that poses serious challenges for health as well as comfort in cities. In the countryside, much of the sun’s energy is absorbed by plants, which draw water from the ground and release it into the air. As water molecules absorb the heat from the air that they need to escape from liquid to vapour, they lower the temperature. In cities, the balance is shifted towards warming. Rainwater is channelled into drains instead of soaking into the ground. Artificial materials store the heat from the sun, especially when they are concentrated in the massive structures of large buildings, and release it slowly back into the air. They may also absorb heat from the myriads of motors and electronic circuits that swarm in cities, though how much all those devices contribute to the warming effects is unclear. Pollution from engines can also contribute to urban warming by creating a local greenhouse gas cloud that traps the sun’s heat. And where tall buildings form ‘urban canyons’, they trap the sun’s energy within them. All this adds up to what is known as an urban heat island, in which there is a marked difference between temperatures in a city and in the surrounding countryside.

The island reaches its maximum at night, between about 11 o’clock and three in the morning, as the city fabric releases the heat it has absorbed from the sun during the day. London is Britain’s most prominent heat island, with temperatures on average four to six degrees higher than in nearby rural areas. It also has...