Now to Calais falling. Germany still turn east in May 1915. I have a harder time seeing Italy staying out of war, but if Italy does, it is similar to above. But if not, Somme was heavily flooded in 1916, so Verdun will likely happen. It could butterfly away Conrads mistake in 1916, but it might not. If a major Russian offensive happens, Russia will still collapse on time in 1917. But war drags on another year. To me, this is why Pemberg is more important.
Calais falling could keep Italy out of the war, especially if Gallipoli is cancelled and Russia is left twisting in the wind. In fact without Gallipoli Russia might make a separate peace because the West has made no effort to help her. The Ottomans then have lots of troops and supplies to use on other fronts, including against Russia in the Caucasus and Britain, if they even try, in Mesopotamia. Italy would have to judge that too. Britain would be tied down fighting on the Western Front in 1915 in several offensives, where they did very poorly IOTL, so they get bleed out against German defenses as per OTL but on a higher scale. Perhaps then Falkenhayn doesn't let Ludendorff wastefully bash against the Dvina line in Autumn. The Central Powers are then relatively stronger in 1916, especially thanks to the Ottomans, who can open up a bigger front against the Russians, drawing off more strength there, potentially turning Persia into a battle ground and causing the British all sorts of headaches.
Germany can then take a more active role in the East or in Italy in 1916.
Beyond that there is the economic consequences to consider; France would have lost the critical farming and coal mining of the Pas-de-Calais/Nord region, plus probably at least 1 million more people, which makes their army weaker in 1916, not to mention bankrupts them faster due to needing more imports of food and coal. The front line is also shorter, allowing Germany to economize on manpower even more, plus anchor themselves on some very useful river lines. A flooded Somme in 1916 in going to cause vast problems for the Entente, as then it gives Germany the ability to keep the Verdun meat grinder going, while also pushing on other fronts; the British are still bashed up from 1915 and probably hasty attacks in 1916 thanks to the panic over the Germans turning the English Channel into a major war area; this too would badly disrupt BEF supply, not to mention that imports couldn't just said down the Seine to Paris for fear of German Uboats and various smaller craft attacking/mining the mouth of the river. French rail lines were IOTL wearing out quickly by 1917-18, so this increased rail burden only increases that rate.
There is no Dover Patrol keeping Uboats from breaking into the Atlantic via the Channel, which makes Uboats that much more effective in 1915-16, which may butterfly away a return to unrestricted submarine warfare.
Plus we aren't even mentioning the vastly disruptive damage to London shipping via the Thames. A WW2 like evacuation of the city and disruption of supply of coal would be very painful to Britain in WW1 because it had less ability to adapt.
In 1915 and 1916 there is much more pressure to attack before the British army is ready than IOTL (pretty similar to the pressure that pushed Paschendaele) which would result in a major series of blood baths for the Entente instead of the attrition of the German army that the OTL Somme was.