CERN is a scientific site developed to create anti matter. Does this have any negative impact on the environment?
CERN is a scientific site developed to create anti matter
The scientific activity at CERN involves many things, anti matter being just one of them. For more information you can read their anti matter page.
Does this have any negative impact on the environment?
No. The amount of anti matter is negligible. Any anti matter produced in CERN is also annihilated at CERN as well and does not "escape" outside into the environment. The CERN experiments require lots of power, so there are power plants that have their own environmental impact. There are also people driving and flying to CERN, but those are indirect environmental effects that are not directly due to the experiments.
Effects of CERN on earth's magnetic field?
CERN has large magnets that are used in their experiments. Their magnetic field is 100,000 times the Earth's magnetic field! But, the magnetic field is constrained to CERN itself and has no effect outside of CERN, and it definitely has no effect on Earth's magnetic field itself.
In addition to @Michael's excellent answer, keep in mind that magnetic fields (just like electric fields) drop off with the inverse of the distance squared (and even faster for e.g. dipoles).
Take for example the magnet used in the LHCb experiment at CERN. It is a large dipole magnet with a maximum field strength a bit above 1T (the Earth's magnetic field is on average about 40-50 uT strong). This puts it in a similar category as the magnets used to bend the proton beams to keep them going in circles in the LHC (which have field strengths of about 7.7T).
As can be seen in the following picture, at a distance of 5m this has been reduced to 20 mT (the line with label 3), so by about a factor one thousand. This is still a thousand times stronger than the Earth's magnetic field, but continuous work for authorised persons is already permitted at 200mT.
Taken from the technical design report of the LHCb magnet.
In addition you should keep in mind that this experiment (and indeed the whole LHC) is about 100m underground, so the magnetic field from the magnets is basically undetectable at ground level, even right above it.
There are, however, other potential effects on the environment:
The accelerator consumes a huge amount of power (about 200 MW at it's peak). Generating this power has an impact on the environment, as already mentioned.
Materials can get activated when being irradiated by the beams. These materials (most notably anything made of iron) are kept under radiological supervision and can only be taken out of the underground area if there is no radioactivity detectable (anymore). One does not want to contaminate the environment with radioactive materials.
Some detector components contain substances which are regulated because of their impact on the environment (because they are greenhouse gases). The LHCb RICH1, for example, contains perfluorbutan (C4F10). The amounts are negligible on a global scale, though.
The beam pipe at the interaction regions consists of Berryllium , whose dust is toxic. Special care is needed when working around it (and probably when dismantling it at some point).
Complementing @Gimelist and @Graipher answers... Regarding your title question...
The CERN magnetic field value it is considered on the CERN ring itself, 5 meters away it is just 20mT... It is just physics... just check this link.
So it is dropping with the inverse cube of the distance. On the top of the CERN is becoming... (100 meters away) around 8uT... so just 5 time less that the average Earth Magnetic Field.
Consider that the Earth Dipole is generated at, minimum, 2890 km from the surface... so if you apply the distance to the field we can detect on the Earth surface (25-65 uT), the dipole on the source should be (average) 962.502.760 million Teslas! (That is not exactly like this, but as approximation)
So a 8.7 Teslas magnet is not affecting, at all, our nearly 1000 billion (international billion, so 10^12) Teslas Earth magnetic field.
Theres the science-y answer and theres direct observation. Direct Observation demands that CERN LHC -IS- supplying an entirely new second magnetosphere. The correlated timing of its appearance and disappearance is intrinsically tied to the operation of the LHC and the SPS.